I can no longer add a product to my collection?

Short answer: A collection takes 9 products at most. Create a new collection, or put the versions of a multi-version product back together to free up places.

Cause

The limit is 9 products per collection, and the figure in parentheses next to the collection's name is its counter. The recommendation is lower still: 6 products, so that the collection stays readable.

The three ways to make room

1. Create a new collection. The simplest, and often the best: a full collection is usually a collection mixing two themes. See How do I organize my products into collections.

2. Put multi-version products back together. If the collection has been separated, the Men / Women / Junior versions count as one product each. Back together, they count as one for all of them — a product in three versions therefore frees two places at once. See How do I separate the versions of a product.

3. Move a product to another collection. Grab it and drop it into a different collection.

Before trying to squeeze in a ninth product, ask yourself whether it really belongs there. A collection of 6 products is browsed; a collection of 9 is endured. The technical limit is 9, the useful limit is lower.

Going further

I can no longer add a product to my shop — the other limit, the one for the whole shop.