"No apostrophe" is technically correct.
But it is a rule that is frequently broken. Such an erroneous apostrophe is called a "greengrocer's apostrophe" or "grocer's apostrophe".
If you do internet searches for those terms you should find a lot of articles.
I sometimes break the rules and use a greengrocer's apostrophe when referring to military aircraft. For example, I will refer to multiple F-16 fighter jets as F-16's.
I do this because sometimes there are different models of F-16 denoted by letter, and I want to make clear to the reader that I mean plural and am not referring to an "s" model of the F-16 (if such a model exists). But I am consciously violating the rules when I do this. I just figure a grammar error might serve to avoid a misunderstanding so I go with it.
On the other hand, if you were to refer to the 1950s as the 50s, it would
be appropriate to place an apostrophe in front like this:
"Back in the '50s they used to...."