ELI5: does the number of 0's in SHA256 over the years strictly increase, or does it depend on how many miners there are?

It is said that to discover a block, the person has to get a SHA256 number that has all 0's for the first 72 bits.

Does this number 72 change and become bigger and bigger over the years (say, every 4 years)? Or does it depend on how many miners are competing?

So that's means, in the year 2010, the number of 0's could be the same as in 2020, if there were as many miners in 2010 as today in 2020.