Yes, QUANTITIES is a Scrabble word!

Letter values in Scrabble US: Q 10, U 1, A 1, N 1, T 1, I 1, T 1, I 1, E 1, S 1.

Is it a valid word?

QUANTITIES is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

how much there is or how many there are of something that you can quantify
an adequate or large amount
the concept that something has a magnitude and can be represented in mathematical expressions by a constant or a variable