You are allowed to swap a maximum of two(2) players per week. If you win your Head-to-Head(H2H) for the week and advance (in 4, 8, 16 team contests), your swaps will be refreshed and you will get two new swaps.
You can see the number of drops available on the Matchup page underneath the player scores or within the Add/Drop window beside the control buttons.
Note: If a player on your team becomes inactive, has a red status, you have the ability to swap them out without consuming one of your swaps.
Also, if you submit a waiver claim and are successful, this will count towards one of your two new swaps for your next week's matchup.
Details on waiver wires can be found HERE.
Matchup Page Player Swap Window