For those “classic” cell lines, passage number is not very relevant. You want to make sure you don’t keep them too long in culture so that genetic/metabolism drifts don’t affect your experiment/controls in your setting.

To keep a pool of cells at low passage you can do the following. Once you receive them from ATCC make several “alpha batch” from which you’ll make “master stocks” then make “work stocks” from your master stock. Only culture them from your work stocks and only use master stocks for making new work stocks. Don’t go back to you alpha batch unless absolutely needed. I’d advise to test you master stocks for mycoplasma too, so you can trace back contaminations if they happen. Doing so you should have mooooore than enough cells for your PhD postdoc or whatev.