I choose by a couple factors
old seeds or seeds with poor germination rates /more++
how many plants do I really have need of, or space for, of one type / less - -
and for me I most often do not re-pot into larger containers so they have to have enough space to survive till they get transplanted to their new spot. I use milk jugs so 6 is about all That will be comfortable till transplant. (I often plant 10/12)
Remember seeds are cheep and you can always weed out the weak ones.
