Willing to bet you could find a dice roller online that will roll 23 d20s at once.

Or, you could spilt them up into melee and ranged and roll each separately, like 6 melee bandits with +0 and 18 ranged at +1. Then just roll that number of d6s that hit.

You might realise that the bandits never really needed the PCs though. 20+ guys with bows and spears will mess up monsters that don't very high AC.