15 buy-ins is the minimum, I like 20-30 unless im at a level Im very comfortable at and have played at for ages. (I generally 4-7 table)

When multi-tabling, in theory you do not have to adjust your BR/Buyin rule - you're simply playing hands at a faster rate. however it may be a nice idea to stick to the rule where you're never playing/riskign more than 10% of your BR at once. The only way multi-tabling can increase risk is if you tilt, then you may spew more chips.