If people call in, do they count as a participant? What if they have audio on, but video turned off?
Zoom counts everyone on a call as a participant, including those without video enabled or those who've phoned into a call, as a participant. As such, if you are nearly at your plan limit and have more people join a call, you may need to upgrade to Zoom's higher plans to enable more participants.
Free plans can include at most 100 people in a meeting. Paid plans can include 100, 500, or 1000 people, depending on the plan tier you choose. If you need more people than that, you can run your meeting as a webinar, which can include up to 10,000 participants.