There is an upper limit of the number of 3D dice that it will/can show in one roll. You can still be rolling more dice & get the result in chat, but there is a limitation on the number of 3D dice that gets 3D drawn in a single throw. I'm not sure if 15 is the limit, but I could believe it. Each one that rolls takes processing power for the dice physics. I don't know what would happen if they allow more or unlimited, my guess at some point it could crash people's web browser, probably affecting lower-end computers first, so this limitation is probably a stop-gap measure. There is not an official setting to change this, there might be a hack or workaround.