Hi Tina,
Please follow these steps to set this up, let me know if you continue having trouble with this. In the plugin’s settings
General > Mode > Product quantities
General > Date range > Custom, 15, days
General > Order statuses > Completed, Processing and any other relevant ones
Limits > Per product > Enable
Then edit the specific product and you will find a new field for maximum products per user. Here you will need to enter the limit of the product. For example, if you need a user to purchase a maximum of five enter 5.
Please let me know if you continue having trouble with this.