How many users can I have in each plan?

That depends on the pricing plan you choose.

The Free plan is limited to 2 users, while both the Team and Enterprise plans can support an unlimited number of users.
However, the monthly price of a Team or Enterprise subscription is directy proportional to the number of users added.

Both of the paid plans offer file attachments and unlimited boards.
Additionally, the Enterprise plan will give you access to a set of rich, exclusive features.

Account owners and admins can manage the plan setting from Account Administration → Plan & Billing details.