Ten items or less? Or, ten items or fewer?

“Pocket Fowler’s Modern English Usage puts it very succinctly:

‘Supermarket checkouts are correct when the signs they display read 5 items or less(which refers to a total amount), and are misguidedly pedantic when they read 5 items or fewer (which emphasizes individuality, surely not the intention).’”

As a general rule, when deciding to use less or fewer:

not as much = less

not as many = fewer

Catherine Soanes explains at length.