You can increase or reduce your number of slots at any time. That way you control how many client slots you have available and only pay for what you need.
To increase or reduce your slots, go to Settings > Subscription and open slot management. You can move your slots up or down in steps of ten from there.
Once you confirm, the new number of client slots is available immediately.
Worth knowing:
Changing your slot count does not change your billing period. The difference, up or down, is simply added to or deducted from your next invoice.