Welcome to FTR!

Your bankroll definitely shouldn't be a factor in how many tables you play. All multi-tabling does is speed up variance. Just add one at a time as you get more comfy with each extra table.

I don't really think you should have a certain winrate before you move up through microstakes. Just post a stat checkup and lots of hands as you play through a level. Make yourself answer hand history threads too. The discussions you get yourself into can be really educational.

Just move up once you don't have any terrible leaks (which you find out by posting a bunch), and once you are comfy with your bankroll. One technique for getting comfy with a higher level is to introduce one table at that level into your sessions while still playing your lower level too (so if you play 3 tables at $10nl then start off with one $25nl table and 2 $10nl tables when you try to move to $25nl).

Good luck man!