How many relays can I connect to one Arduino?

Literally threes of people have asked me this question, so I decided to find out. I love these 8 channel relay boards, but how many can you attach to one Arduino and what's the best way to connect them? Oh. I know I repeated the DuPont part. I couldn't bring myself to re-edit :) Let's just say I wanted to drive that point home.

Links below are affiliate links and may earn a commission for the channel.

USA
8 Channel Relay - https://amzn.to/2J4EElK
16 Channel Relay - https://amzn.to/3mvUGmc
DuPont Crimpers - https://amzn.to/3h7tg51
DuPont Connectors - https://amzn.to/34vhAnz

Will It Blow Up - Episode 001 - Maybe this should be a thing?