There will be as many levels as I can fit, once I get the dang password system working. Restoring a saved game and loading a level are basically the same idea. I want to compress savegames to make entering them as text, in-game, tolerable. Levels in ROM will basically be pre-typed passwords, and I only have to do the one system.
That's the same reason the enemy placement hasn't changed since the first release, even though some of them are inaccessible without cheating with the second controller. (Or rocket splash damage.) It is possible to have hundreds of enemies in one level. There's a rudimentary infighting system, if you get one type to shoot another. But they haven't moved because I don't want to code something I know I'll toss out later. I'm already replacing the wiggly rockets when you fire along a diagonal.
The deeply amusing side effect of a password system is that levels might work between platforms.