Buses have cavernous storage spaces beneath the passenger floor, atleast here in finland. You can fit practically as many bags as you manage to carry in there.
I think it's fair to point out that the buses with cargo space are mostly used for long distance routes. Local traffic buses used by most cities don't have dedicated space for luggage.
Here's an example: https://arpeco.fi/wp-content/uploads/2020/01/20191231_143830... Can you see those big hatches? They're almost the depth of the bus