[1024]

On the other hand, are more than 500 voices ever going to be needed?

Old Yamaha XG software synths could play / process up to 1024 (!) voices at the same time (selectable from 32 up to 1024 in the S-YXG control panel)
And that was ~15 years ago when everyone had slow Pentium III PCs

For xmp-midi, it's best to set a hard limit of 1024 voices max.

Then set the default (soft limit) to a reasonable value (256 voices)

Finally, add a setting to increase the voice count to 512 and 1024 (for advanced users)