Use an apostrophe whenever you are talking about more than one number, letter, or symbol.
Plural letters need an apostrophe.
four B’s – I need four B’s to write my full name.
seven X’s – I used seven X’s in this TIC TAC TOE game.
five E’s – The lady got five E’s on Wheel of Fortune.
Plural numbers also need an apostrophe.
six 9’s – There are six 9’s in this UPC number.
three 6’s – I rolled three 6’s while playing Yahtzee.
two 3’s – My phone number has two 3’s in it.
We also use an apostrophe with plural symbols.
#’s – Did you count all the #’s in this report?
$’s – Can you use fewer $’s in this sale ad?
?’s – There are too many ?’s in this paper.
Always use an apostrophe when writing plural letters, plural numbers, and plural symbols.