It's like saying a canine over 20 pounds is a dog, but anything less and its not. An animal weighing 20.01 lbs would be one thing and an idential twin weighing just slightly less at 19.99 would be something else. Planets are built from planetesimals, and will therefore have a continuum of sizes. You need to look at some kind of phase change to distinquish between them rationally.

1) Big enough to hold hydrogen as a gas? - 4 planets

2) Big enough to have a significant atmosphere? 6-8 planets. Problem very position dependent. You also have to define significant atmosphere which again can be arbitrary. Titan would be a potential planet but similar Ganymede only 5 AU in would not be.

3) Big enough to be round? 12+. Also position dependent, but not as much. Also there is no reason why a planet made mostly of gas should count and one made mostly of ice intermediate in density between some gas giants and teresstrial planets should not.

Regards,
MArk