25
All of them.
get all the characters
sonic and toad
play 1000 vs matches to unlock character 25, Mr. Game and Watch. There are no characters after that.
Technically yes sheik is a separate character making 26 characters.
There are 25.
If by "Koopa" you mean Bowser, he is playable from the start. "Purin" or Jigglypuff is one of the easiest characters to unlock
No
No he is not.
No.
You can't get sonic in SSBM only in SSBB and in SSBB you get him at the subspace emissary near the end