Yearly vs Monthly Subscription
You can pay on a monthly basis. You will be billed each month (per user/per seat) and can cancel anytime.
Yearly billing provides you with a discount compared to the monthly subscription.
Number of Seats
There is no minimum for the number of seats (users) you are required to have to use Klaus. Your subscription is based on the number of seats you use, and you are able to add more seats at any point.
You can cancel your subscription in Settings > Subscription. This will stop the auto-renewal of your payment plan.