Hey Shero,
A good BRM to start with in the Micros is a conservative 25Buyins.
NL2 - $50
NL5 - $125
NL10 - $250
And so on...
You can sway either way of this a little, give the higher limits a shot if you're close but providing you stick with this roughly then you'll be fine.
You can read more on BRM in the start of This Article.
As for taking Multi-Tabling, I personally don't think matters too much with your BRM. As long as you're only playing an amount of tables that you can comfortably play your A-Game on then it shouldn't be a factor, just be sure to implement a strict Stop-Loss as going on tilt when playing more than 1 table can be devastating to your BR!
Hopefully that points you in the right direction, but if not don't hesitate to give us a poke