More accurately anything that is not 0 is true.
So 1 is true, but so is 2, 3 … etc.
You neglected to say which version of C you are concerned about. Let’s assume it’s this one:
http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf
As you can see by reading the specification, the standard definitions of true and false are 1 and 0, yes.
If your question is about a different version of C, or about non-standard definitions for true and false, then ask a more specific question.