There's no limit on the number of people you can invite to a board.
There's no limit on the number of people who can simultaneously use your boards.
There's no limit on the number of people that can simultaneously access your boards.
Note: It’s important to keep in mind that depending on the number of people using a board, it may become harder to organize the cards and conduct a proper meeting.
If there are too many cards and people are adding/voting at the same time, it can be challenging to control the meeting.