If I buy a Virtual Console game on the 3DS for the NES, and if that game is 2 player or more will it work? I am not sure if there are any 4 player NES games on the NES... will single copy play work on all of them?
This depends on the nature of the multiplayer. There are basically 2 categories of NES games that I know of:
Simultaneous play: If the game allows 2 players to play at once, then the game will support Download Play. You would need 2 3DS systems in close proximity to play this way. In this case, only one player needs to have a copy of the game.
Alternate play: Many NES games support "multiplayer" by allowing one player to play at a time, but tracking progress separately. For these titles, both players will use the same 3DS system.
Some GameGear games also support local multiplayer. You would still need both people to be nearby to play. For these games however, both players will need a copy of the game.
As far as I know, only NES games and GameGear games support multiplayer. Virtual console games for other systems do not support multiplayer in any way.