There was an article in the old TSR Dragon Magazine eons ago, and was reprinted in one of the Best of Dragon anthologies (vol 3 IIRC?), that was entitled How many coins in a coffer? It was a simple article that showed volume calculations and coin sizes and volume ratios down to a single coin. Granted, it was based on 1st-ed coinage descriptions, but I had no reason IMC to think any differently about the coin sizes in 3rd ed.
At any rate, the bottom line of the article stated that any given container would fit "about" 4 coins per cubic inch of space. For example, in a small chest measuring 12x6x8 inches, that is 576 cubic inches, which would hold in total about 2304 coins. It went on to compare other space values of other containers and extrapolate how many coins it would fit (ie. a backpack could be expected to hold a wizard's master spellbook, which might reasonably be of a size X by X by X, therefore a backpack's interior space would be Y cubic inches, which finally meant that one could stuff 4Y coins into that backpack.
But of course stuffing all that coinage into a single backpack would exceed the weight capacity of said backpack... Anyone who's seen the George Clooney/Mark Wahlberg movie Three Kings will know what happens when you stuff a load of GOLD beyond a bag's carrying ability... But in terms of your original question, how many physical coins fit into a physical space, that is the simplest calculation to reach the answer.