Maximum of 100bb, since stars doesn't have deepstack tables anymore. Buying in for a shortstack is a viable option if you plan is to be a shortstacker on the cash tables but if you have an edge on the competition then you should buyin for and remain auto topped up to 100bb's.
The size of your stack will generally impact the strategy you should be playing by, the deeper your stack the more post flop play will be involved and the more streets of play so will involve a higher skill level to play optimally.
Personally i tend to flag shortstackers as many can be a hit and run type, double up their shortstack and leave so would give them less action without a stronger hand (unless they also a complete fish, in which case I enjoy taking away their shortstack multiple times if they stick around and they don't run like god on me rofl)