There is "dead space" (i.e. justair) in between gumballs. If they were somhow liquified, then we could just divide the total volume (~900) by the volume per gumball (~.5) to get ~1800.

One approach would be to take a rough average between our first guess (1800, or 1728...) and the following:

Imagine 1 (in^3) cubes. We could fit ~900 in there, so the average would be around 2350.