40 or so is a good limit on waiver/free agent transactions. Enough to make changes when you need them, pick up some hot hands from time to time, but not so many that you can start churning. Normally, I'll have about 25-30 transactions in a given year. But some go right up to the limit.
I wouldn't limit trades unless there is collusion, in which case I'd recommend you get out of that league. Normally, trades have to approved by either the commish or the league (or both), so I wouldn't limit them.