Entire lineups don't really lock, indiv players do, and IIRC it's on the minute, or possibly 5 mins before gametime. You can set lineups daily, or just do it for the whole week. Be warned, if you make a waiver wire move it will reset your lineups, so always confirm lineups after making changes.
No maximum games, but Wed & Fri tend to be the full days, so you may have to make decisions about who to play/bench there. Yes, it varies how many games a player has in a week, so that's just luck of the draw.