I'm new to blockchain and trying to figure out what determines/controls the transactions that are included in a new block. 

If I understand correctly, miners pull from pending new transactions in the mem pool. But;

1. What determines if they choose 1 or 10 or 50?

2. What determines which transactions are chosen? Most recent? Highest fees?

3. Is there some reason for not choosing as many as possible to get more fees?

4. Is this something you configure according to some criteria in your mining software or some thing determined by your pool?

Thx