Is most software designed for Windows? Is it hard for Mac users to find what they need?

How hard is it, generally, to find a Mac alternative to a program written for Windows? Is it very uncommon for a program to be designed for both Mac and PC? Are mac users ever stuck with the fact that they just can’t do things that require certain software on their computer because it’s not a PC? And if so, how often are they stuck like that?
These could be totally inaccurate impressions. I don’t know, because I’ve never had a mac. I’m thinking about getting one, though, so I need to know these things.