MTG Player lets you take decks you've created in the Manager and play them online against your friends. It can theoretically support 100s of players per game. However, I wouldn't recommend trying it. I've been using it to play 3 - 5 player games consistently.
The player doesn't try to interpret any of the card rules. It's basically a virtual card table. Actions such as managing life counters, turn order, play order, etc. needs to be managed manually by the players. As such it's crucial that all players have live voice chat during play. It's funner that way, anyway. Skype and Ventrillo are excellent choices for this.