You beat the first 20 then it ends and unlocks the rest. There is another thread about wii play tanks, do a search. People said they've beaten all 100. I think I've gotten to 17 with two players, my friend said he got to like 30.
I want wii play just for this game, but I'm gonna wait till people start selling used copies for cheap on ebay. It's very worth buying if you need the remote, otherwise not really. The price it goes for without the remote on ebay currently is not worth it in my opinion, it'd be more worth getting a good gamecube game for the same price.