Adding extra accounts

Hello there:

Today I became a paying Zoho customer and added six users to my account. I was correctly charged $15 for the month.

I am still experimenting and will add another 20-plus to this account.

I want to make sure that if I add another 20, I will be charged for the additional 20, and not for 26—because if I alter the upgrade form to read 26 from the present 6, it indicates my charge will be $65. Will the invoice that next comes be for $65 or will it be for $50?

I apologize for the basic question but I am still getting used to the way Zoho is structured.

Kind regards,

Jack