I have several numbers as we are FI but not RE yet. Some have real thought to support one thing or another, others are just gut check, etc.
Number of kids: 2 (this became very important to us)
College fund of $100k per kid by age 10 (on target by end of 2017 and invested)
Work to age 50, a personal, mental thing I have, part time contracting after 50 maybe
Current/proposed retirement budget: $65k/78k per year (supported by rental income)
SWR: 3.5% or less if RE at age 50 (on target, not including small pension or SS)
Net worth without rentals or primary residence: $2M (in good shape)
I'll probably work longer than I need if looking at pure retirement budget, but ok with that. When it's time, I'll have belt and suspenders plus SS in the future. ;-)