No more than 100 people may be ashore at any one site at any one time. It's an IAATO rule and it applies across the industry, to protect the environment and keep numbers down around wildlife. The critical thing to understand is that it limits how many people stand on land at once, not how large the ship is allowed to be.
This is the most misread rule in polar travel. People take it to mean they need to book a ship carrying under 100 passengers, which is wrong and cuts them out of most of the market for no gain.
Ships up to around 200 handle it by splitting into groups and rotating, so everyone gets their turn ashore. There's also a quirk worth knowing: passengers out on an off-ship activity like kayaking don't count toward the 100, and a few operators use that to get everyone off the ship at once.
Before every landing you'll get the IAATO briefing covering conduct and what's allowed, and guides scout each site first for both safety and wildlife density.