The number is growing exponentially. More and more businesses are moving over to mac based environments.
As for the differences, yes their are differences. Things behave slightly differently. You can't say windows is right and mac is wrong, as who is to say windows isn't wrong on how things are done? With very little effort, you can get used to using both mac's and PC's. I used both daily for around 2 years.
As for one button mice, they are what came with the older macs (PowerPC). Instead of right click, you control+click.
As for deleting, to get a normal delete you do shift + delete if I am correct? Hard to remember, these things just come naturally.