Char a[10]. How many characters can I input in this string

Answer 1

That's also a character.
You can also put a regular one into the last slot, then you have an array with ten letters, you just can't use it as a string in that case.
(Whoever's so quick with the downvote colt here, should try to think a bit harder next time.)

Answer 2

10.

Answer 3

It is a char array in any case.
As soon as you put a '\0' in there, be it at the end or someplace else, you can use it as a string, with printf or something.
Erase the 0, you can't.
Re-insert the 0, you can again.
Try it out for yourself!