I don't think there is a limit (or, at least, no one has said they've run into issue). At one time, I had six accounts on the iPad - I'm sure I'm not the one with the most. So - load 'em up!
I have 5 different email accounts so i want to know if there is a limit
There are many ways. You can load files using iTunes, download/open them from cloud storage or e-mail files to yourself. There are some apps (Goodreader, iFiles, etc) that make it so you can connect your iPad to the computer (using FTP protocols), while IFiles allows you to make a network drive so that the iPad can be used as another storage/transfer device. Then, there are photo apps such as Flick or Bump (among others) that allow you to wirelessly transfer files using a web browser or small server programs installed on your computer.
I've used all of the methods mentioned above and I am sure there are other ways. Just as I am sure others will chime in with the ways they move files around. it really comes down to what you find and become comfortable with; what works for you.