regarding number of macros: there are 36 macros available without an addon.

Trinity Bars 2 is a replacement of the default action bars. It allows UNLIMITED buttons any of which can be configured as a macro button (with a length of 1023 characters)

I have every action configured as a macro (I like the flexibility) so I figure I currently have around 40 macros on my level 23 priest

regarding how the script works.

it is simply a command in the same way as /cast, /use, /dismount etc except that it allows you to execute native lua code

for more info plese see here http://www.wowwiki.com/Making_a_macro