It says that the premium plan can have up to 1100 subscriptions. Does this mean up to 1000 customers who have subscribed, or up to 1000 products set up in subscriptions/rules that I've set up in my shop?

This means that you can have up to 1100 active subscriptions in the app. The app has no limits on the amount of products you accept subscriptions on or how many subscription rules you can set up.
A subscription is represented by a subscription interval in the order. This means that if the customers place an order where they subscribe 2x to product A with a weekly interval and 1x to product B with a monthly interval in the same order, then the app will split this order in two subscriptions. One will be a weekly subscription with 2x product A and the other one will be a monthly subscription with 1x product B.