A bit of both I would guess. Yes your wheel should have an even number of bearings-ie the same number on both sides of the wheel.

the genious part(for a beginner)- Stealing a bearing from another wheel. That is standard practice for many garage mechanics- but you figured that out on your own-Kudos you have the makings of a future expert mechanic

the beginners luck- not all wheels use the same size bearings- if your wheel is spinning true now you most likely stole a bearing that is of the proper size. If you run into problems with the wheel in the next couple rides I would take all of the bearings out and get new ones from you LBS and replace them all with the correct size. I think you will be fine tho.