it all depends why you play, whats your take on depositing more money, how fast you want to go through microstakes and also your personal properties and obv. the type of game you play with rule: higher variance more conservative BRM, how many tables.
If you don't mind jumping stakes in matter of days or hours you can play a style with a lot of shot taking and small BI restrictions, if you already play decent stakes or/and poker is big part of your income, you aim more for stability and being able to withstand all kind of downswings without having to jump stakes or getting too nervous;)
so as with almost everything in poker, it depends. experiment, have fun and figure out a way thats fits you the best.
If even so you would like a cold number I would say till nl25 number of 20-25Bi normally is enough, later really depends on you, your win-rate and stuff I've mentioned before.
gl!