I'm buying bills so I have not looked at notes (1 year+) but I believe you buy them (notes) in $1000 increments, $1k is 1 bond. For T bills they are sold at discount so you pay a little less than $1000 per bill and get the interest at maturity.
Here's a bizarre twist at Vanguard! Beware if you buy treasuries in your IRA! You need the money to purchase the treasuries in your "settlement fund" which is the Vanguard Federal Money Market settlement fund. BUT there is also a Vanguard Federal Money Market FUND that is a mutual fund!
To get the funds from say a mutual fund in your IRA into the settlement fund, you have to SELL not EXCHANGE!!! I did an exchange, sell sounds like a taxable event even though you are moving within the IRA so my money went to the FUND not the settlement fund. I had no money in the settlement fund this afternoon and called to ask why since the transaction on Wednesday was complete. So the money in the Vanguard Federal Money Market Fund (mutual fund) had to be sold today and put into the Vanguard Federal Money Market settlement fund.
Why in the world is this so convoluted? Luckily I have time to do the purchase of Treasury bills on Friday but this was quite a surprise and if time was a factor I may have missed the auction.
It would not be a taxable event if you sold in the tIRA. We did the same thing and sold most of our bond funds. The money went into the tIRA settlement account. We bought T bills with a portion of that settlement fund and are waiting to see what happens in the next few weeks.