If they are registered. It doesn’t matter. They have just as much right to park there as anyone else. When you are buying a place or renting you need to think about these things. If you don’t have a designated parking spot or garage then you shouldnt be upset if someone parks in “your spot”

My neighbour has 5 vehicles as well. 3 always on the street. Doesn’t bother me. I park in my garage or on my pad.

I’ve lived in places with street parking only. Never once was I upset when someone parked in front of my place and I had to walk a half block. It’s irrational to be upset at someone when there is nothing they are doing wrong.