Which are the factors that decide number of passes for a compiler?

There are many. I have encountered compilers with as few as 2 passes to as many as 63 passes (this number of passes were necessitated by the very tiny magnetic core memory of the machine). I even encountered a compiler that the programmer that wrote it claimed was a 1 pass compiler, but on detailed examination all he had done was "piped" the output of pass 1 to the input of pass 2 (instead of writing it to an intermediate file between passes), so it was actually a 2 pass compiler.

I have also encountered a language (the US Navy's CMS-2) that has both syntactic and semantic ambiguities that made it impossible to write a compiler (no matter how many passes) that could reliably compile all code given to it, to do what the programmer intended.