“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.