UI design plays an important role
that number comes from UI design: How much space to allocate on the screen for the level number.
I would like to underline this fact even more. This is true not only for level, but also for other stuff such as HP, MP and other stats in general.
This is even more apparent when trying to find a formula to balance your game. Such a formula is comprised of so many different variables that not only does it have infinite valid solutions, the space where said solutions lie is N-dimensional.
That being the case, how can somebody define a starting point? Well, UI design plays a role in that. If you have enough space for 3 HP digits, you put just 3 of them, then repeat the same thing for all the other stats and restrict said space of solutions to a much smaller portion of it.
From UI design and balance to levels
You might wonder what all this has to do with levels, since I only mentioned stats so far, but it plays a very important role when defining how many levels you put in your game (or, at the very least, it should).
Do you think that gaining a mere 10 HP when you already have more than 800 would make a difference? Probably not. Levels should bring meaningful changes, not always major ones, but at least relevant.
What would be the implications of a game only having 20 levels?
As long as any level up is meaningful to the player, you can have a game with only 20 levels, especially when you keep it short, it would be much better than having 99 levels with 79 of them being meaningless.
Carefully designed level ups and breakpoints can definitely depict your game design as more professional and thorough, however, this does not mean that fewer level ups are always the best solution.
It might be that your game has a very slow progression leading players to achieve meaningful changes every 10 hours of gameplay (whether it is by farming experience or not is irrelevant here). According to what I previously said, allowing the player to level up every 10 hours might be a good solution, but it might not be like that.
Many games are not very long, and even for long games, 10 hours for a single level up might be too much. Players would not feel rewarded and levelling up becomes a distant goal, so far that it might even feel unrealistic.
Exploring multiple solutions to the slow-progression problem
Solution 1: Getting rid of levels, use other means to strengthen them.
This is a feasible solution: when levels get in the way, get rid of them and allow players to have mid-goals like crafting equipment parts, unlocking skills through quests and/or items. Put something in your game capable of entertaining the player while keeping him/her focused on the next small goal. Clear and close goals are far more engaging.
Solution 2: Add more levels to split the bigger goal in smaller parts.
Another solution would be that of adding more levels. They are not going to bring meaningful changes every time, but they help by keeping the player hooked. It might look like this goes against the idea of having meaningful level, but it does not. Levels are still meaningful because they still serve a purpose: they give players short-term goals, acting as a de-facto progress bar between the previous breakpoint and the next.