Thorns does stack, I just tested it in Java edition 1.15. 2. These numbers line up with the partial description of its mechanics given in the wiki. It stacks.

Is thorns 2 A max?

The maximum level for the Thorns enchantment is Level 3. This means that you can enchant an item with up to Thorns III. The higher the level, the more powerful the enchantment.

Is thorns worth it with mending Minecraft?

I'd say it is worthwhile. It affects all damage directly dealt (including skeleton arrows) and Unbreaking 3 does seem to help. Also, if you add Mending the durability cost could potentially be negated.