You generally can't name things with a number, because it makes it ambiguous whether a name is a name or a number. The best example is with object type names, where they can appear in expressions e.g.
Sprite.X
If you could call a sprite "34" then it appears as
34.X
which is difficult for the parser to figure out what you mean (since 34. is a valid floating point number). That doesn't exactly apply to animations, but for consistency and future-proofing the same principle is applied nearly everywhere. Workaround: use names like "a1", "a2", "a3"... and set the name "a" & number.