Why did my Nikon FM2n frame counter jump, and how do I know if I reached the end of the roll?

It sounds like you are expecting the frame number indicator to act like a camera that uses a motorized film transport to pre-wind the unexposed film onto the take up spool inside the camera as soon as you shut the film door after loading the film. As you take images the film will be wound back into the film cannister by the motor. With such a camera the frame number indicator will typically show how many unused exposures remain available.

But you've also indicated that You have a manual film advance lever. Cameras with manual film advance levers don't pre-wind the film onto the take up spool and wind it back into the film cannister as you shoot. At least I've never seen any camera that does that.

Cameras with manual film advance levers instead of electronic motors that advance the film only roll the film onto the take up spool after the film has been exposed. The frame number indicator counts up as the film is advanced. It can't count down because the camera has no way of knowing how long the film still inside the cartridge is.

Your comment to a different answer:

Okay so now when I try to advance it the lever wont move. It is stuck on exposure 31 out of 36. Is there a way for me to rewind and unload the film somehow, and just develop it hoping something somehow works out? I'm still new to this all, but the past roll of film I shot and developed went very smoothly.

If your frame indicator started at "0" and increased by one frame each time you advanced the film until it reached "34", then started counting back down to "31" and got stuck there, something unexpected has happened. It's possible the film got folded and starting loading on the wrong side of the take up spool and then started unwinding as you continued to operate the film advance lever.

Here's what you can do.

• Check the film release button that you normally push in order to use the rewind crank to roll the film back into the cannister. It may have been accidentally pushed, allowing the take up spool inside the camera to "freewheel".
• Since the film advance lever is now stuck, press the release button and crank all of the film back into the film cartridge. Try and "feel" if the film is actually being wound back into the film cartridge or if the lever seems to be turning with no resistance at all.
• If the frame counter is not counting back down as you rewind the film back into the film cannister, then assume the film is broken or has become unattached from the film cartridge and only open the camera back inside a TOTALLY DARK room. Any light whatsoever will fog any length of film that has not been rewound into the film artridge.
• Operating by feel, try to gently pull the film off the take up spool inside the camera. Repress the manual release button if needed to allow the film to unroll from the take up spool. Try as much as possible to handle the film by the edges so as not to get fingerprints on the exposed frames in the center 24mm of the 36mm wide film strip. The edges of the exposed frame are only 0.67mm inside of the inner edge of the sprocket holes.
• Without exposing the film to any light, load it into your developing tank and proceed as you normally would. If you have two lengths of film on either side of the break, it might be easier to place one in a light tight box while you load the other into your developing tank if it's a small "single roll" type of tank.