When you remove a member during the billing period, we will not charge you the full price for this user. We'll add a prorated amount in credits to your account as a refund for the time you pre-paid for but did not use.
You are in the Company plan and have 6 users in your team. This means you are paying $20 x 6 = $120 monthly.
Say, you decide to remove one member in the middle of your billing period. In this case, we will refund a half of the cost you paid for the user, i.e. you will receive $20 / 2 = $10 in credits.
This means you will pay $10 less next month.