The general rule is that you use fewer for countable things (the noun take an ‘s’) and you use less for uncountable things (this thing cannot take an ‘s’).
If you only want to buy 8 items at the grocery story you can use that checkout counter. It is for 10 items or fewer (items has and ‘s’ and I can count it individually–so I use fewer).
I have a lot less money now than I did before I went to Las Vegas! (I can’t put ‘s’ on money–so I use less).