There are around 40 of these characters to collect, although it should be said that you can finish the game with the three that come in the starterpack.
Skylanders Swap Force StarterPack looks like it will come with the game, a new portal, Series 3 Stealth Elf and Swap Force characters Wash Buckler and Magna Charge.
This year for instance players will need to buy a new Portal starterpack for Swap Force as the mix and match characters require some new tech under the hood of the peripheral.
There is the introduction of a third iteration of some of the characters, like series 3 Stealth Elf, as well as the issue of having to buy a new StarterPack this year as the old portal will be incompatible.