Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Confirm you are talking about 365 Home.
.
365 Home allows you to "share" with up to 5 other people (used to be 4). Each of you can install on up to 5 computers at the same time.
.
Buying more 365 Home subscriptions and using the same email account to activate them will only extend your subscription end date. You cannot increase the number of people allowed on the single 365 Home account.
.
Note: 15 (or actually 18) users suggests that you are using it in a "non-home" use, business or non-profit type situation. 365 Home is not approved for that sort of use. You probably should be using one of the 365 Business license types, one for each user.