27 unique levels. 40KB minus a handful of spare bytes and some unused code. The max the NES can support without mappers. Modern NES homebrew and demoscene can do fancier stuff with this budget given the extra decades of learned tricks, but for the state of console gaming in 1985, SMB1 is damn impressive.

Also remember all of that was ROM, the NES had a mere 2 kilobytes of RAM for all your variables and buffers.