Shopify has predetermined hard limits set within their Liquid code endpoints to reduce load on their infrastructure and give all us merchants the best performance possible.
One of these is the ability to return more than 1000 products in a query.
Instant Brand Page dynamically queries a stores Products to get discounting and availability information for each brand. So if you have more than 1000 products it will not be able to look at them all...maybe
To work around this limitation, Instant Brand Page will do multiple queries based on each of the stores Collections. So as long as all Products are a member of a Collection and each Collection has less than a 1000 products then no problem!
For example: A store has 2000 products, 1000 t-shirts and 1000 hats. As long as each group is contained in a Collection of say 'T-shirts' and 'Hats', then Instant Brand Page will be able to check all of them.
Moral of the story: Make sure all products are segmented into smaller collections and you will be fine (it also makes good organisational sense!)
Read this to see how to create a Collection
To get the best responsiveness for your Instant Brand page you can also set the following settings for your page
Out of Stock Brands = Checked
Discounted Brands = Unchecked