japanese language question/translation?

Let's break this down.

「とても すくない」: "very few"

と: in this case, this is a particle meaning "and", not the と that follows a quote

「ほとんどいない」: "almost none" (rare is also a suitable translation in some contexts)

の: this isn't the nominalizing の, this is the possessive の

中間: middle (can also mean "between" in some contexts)

くらい: around/about/approximately

では: combined particles of で and は

あるまいか: a classic way of saying "probably (not)" in a similar vein as saying ではないでしょう?, but perhaps with a closer meaning to "might".

So, combining all that together, you would get:

"very few" and "almost none"'s about middle might

Or, interpreted into better English:

Might be somewhere between "very few" and "almost none"

(or alternatively: Probably somewhere (around) in the middle of "very few" and "almost none")