There is not an easy answer to that question. It depends on how much sensitivity you want your receiver to have and how big you expect your blockers or interference signals, to be. The LNAs also present to make up for the losses of other components in the recevier signal chain such as filters and switches. I would suggest starting by putting in enough gain to achieve the sensitivity you need. Then Figure out what happens if you apply a blocker. That may force you to use a second stage LNA which has a bypass path. So in general, I would say that it's about figuring out how much gain you need while still ensuring that you won't overdrive or damage the receiver.