The info at
this link answers most questions on the details of constructing conditioned crawlspaces. This would be a good guide for your discussions with whoever is going to do the work.
Most I've talked with are talking about only 2 to 4 vents in the crawl space. Enough to create a positive pressure. Don't see how this will make the crawl space feel exactly like the house. Most individual rooms have 3 and 4 vents.
Maybe I should start with the 3 or 4. I can always add more vents as I monitor the situation...right?
See the options in the document at the link above (page 4). Note that the "transfer grills" mentioned are simply an opening or duct which allows air to flow from your crawlspace to the house. Any of the methods shown on page 4 will work, I'd probably favor Method D. Regardless of the method chosen, you need 20 CFM of supply air per 1000 sq ft of crawlspace. The amount of air that comes out of each register depends on the pressure supplied by your AC fan. If your system is designed properly now, you could figure out the CFM per inch of register area by finding the CFM for your AC fan (at the speed it normally runs in your house, some have multiple settings) and then divide this by the sum of all effective area of all the registers in the house (ones that are partially closed= estimate effective area). If this sounds like too much work, you can just use a rule of thumb: in a properly designed system, a 4" x 10" register supplies about 30-80 CFM depending on fan speed. So, if your basement is 2000 sq ft, you'd need 40 cfm of supply air = about two of these registers. Buy ones you can close off in case later you determine your system is working really well and you want to reduce the flow to the crawlspace.
HVAC company says they normally put the vents in the trunk lines to spread out the air flow. They are talking about using something like dryer vents so they open when system runs and closes when it doesn't. Are these type vents appropriate?
You could install such a thing (a "backflow damper"), but it's really not needed and it's another thing to fail and get stuck in the closed position someday. Regarding the location of the supply air registers-- you'd like to establish a flow pattern in the basement from the supply air registers to the return air point(s) (see below). The air should leave the registers and cover almost all the crawlspace before reaching a return air grill/opening. If it's convenient, then spreading out the supply air registers on the branch lines is okay. What you want to avoid, however, is starving a room register by installing one of these crawlspace registers. If you've got a room that gets only marginal cooling already (maybe a room that has a lot of windows and gains a lot of heat, supplied by a single 4"x8" register fed by 40' long 8" dia duct), you'll make matters worse by installing another register on that branch.
They are not planning on an air return....again because they are planning on just creating a positive pressure...allowing for air to escape up thru the floors or other unseen cracks and crevices.
Not sure I would want that air returned into the house.....until I have more of a handle on the mold situation. Am I wrong about this?
Yep, I think you want that air coming back to the house, otherwise you'll be pumping nice cool air out into the neighborhood all summer. If you don't deliberately build in a way to get the air back to the HVAC system, then much of it fill wind it's way outdoors, and the remainder will come up through your floor into the house. Every cu ft that goes to the outside of your house depressurizes your home's envelope, which will cause air to leak in somewhere else (driving up your AC bills, bringing in pollen and outdoor pollutants, etc). Taken to extremes (unlikely in this case), this depressurization can even cause problems from backdrafting natural gas flues with accompanying CO "issues." See figure 5 on pg 5 of the linked doc for a diagram of a transfer grill--this (these) would be best placed in a hallway or other spot where there will never be a door closed between it and the home's return air plenum. Or, if the return air plenum itself is in the crawlspace, you could just put the grill there. Again, you don't want the return air grill close to the supply air registers in the crawlspace--this would create a "short circuit" loop and the really damp air in the crawlspace won't be picked up in the return air duct and sent to the AC evaporator coil to have the moisture removed.
Note that if you develop positive pressure in the crawlspace (relative to the rest of your house, perhaps because you didn't include transfer grills, etc), it means the supply air registers in the crawlspace will be pushing against this greater pressure. The "backpressure" will be less than this at the other registers (in your rooms) and that's where the air will flow. So, not providing a way for the air to get out of the crawlspace and back to your AC unit will reduce the amount of air that gets dehumidified through your AC unit (the main reason you're doing all this work). I think you want almost neutral pressure down there--virtually the same as the rest of your house.
Regarding the mold: I'm not an expert. I do know that mold spores are everywhere all the time. But, if this were my project, I'd probably wet down any visible mold with one of the commercial mold eradication products (available at Lowes/HD), then I'd count on the low humidity to prevent any future problems. The Scotch Filtrete furnace filters claim to capture mold spores, maybe I'd use those over the return air grill to capture any mold spores for a year or so. Again, none of this is based on any thorough research of reputable studies (to the degree that's available at all in the mold biz!). Know that if you don't provide transfer grills and instead just let the air find it's way into the house through cracks and crevices, it will still be the same air (including any mold or dust from the crawlspace) as if you'd provided a dedicated duct, except you won't have any way to filter it (if such is your wish). It might not be a bad idea to put a fan down there to stir the air up for a few weeks after you get this work done, that should help dry the place out a bit faster (the circulation will help the air pick up more water from the floor, etc between cycles of the AC).
Also, as the wood dries out the floor may behave in "new" ways. If it's plywood or OSB you probably won't have any problems, but if you've got planks anywhere . . .
Radon: See figure 13 in the attached document. If there's any chance you might have a radon issue in your house, it's smart (and cheap) to put in a manifold for removing this soil gas. Just a few trenches in the dirt about 6" deep lay in some 4" perforated pipe all connected to a single pipe sticking up at a convenient spot for a vent flue to be installed later (if needed). Cover the pipe with coarse gravel, then lay your plastic all over the dirt floor and continue with the work. Then, after the the crawlspace is done, I'd close all vents to it and measure the radon level for a few days (kit: about $20). If there's a problem you can buy the fan and run the flue line out through a wall to a point above the roof line.
Again, be sure to do your own research so you understand exactly what you are trying to achieve, and recognize that my opinions are just as "authoritative" as any other random gurgling you'll find on the interwebs. It sounds like you are well on your way.