A few guppies short of an aquarium.

A few inches short of a foot/yard.

A few kernels short of an ear.

A few kopeks short of a ruble.

A few links shy of a chain.

A few measures short of a staff.

A few open splices.

A few peas short of a pod/casserole.

A few pickles short of a jar.

A few pies short of a holiday.

A few planes short of an Air Force / hangar.

A few points short of a polygon.

A few revisions behind.

A few sandwiches/apples/ants short of a picnic.

A few screws loose.