Add option to set max / min value in number column

I’m in the process of implementing a seminar signup.
I need to restrict available input options for the number column, but don’t want to switch to a single select (not sure if I can sum it in lookup formulas if the number is in a select option as text).

Min, Max, or Sets would be useful. Set being a user defined list: 2,4,7
Maybe even steps, as in 10s: 0,10,20,30,40,50 etc

Not sure if all of these are practical or really that useful, though.

My use case for sure would be: set the max per registration to the currently available seats (already being calculated in another column)

Single select values is one option.
A link to other records column is another one:

I can use a formula and calculate stuff, but I can’t set the upper limit like that I think. In one table I have the column where free slots are stored (gets lower as people register), and in another table I want to have a number that has a dynamic upper limit dependent on the (still) free slot count.

Maybe I’m just not seeing how your answer would cover that case?