There is no maximum limit for number of possible variants on a product. However, there is an upper limit 1000 variants per batch.

In the below example, I tried creating a total of 2000 variants at once for a product.

Variants are created in Odoo as a combination of Attributes and number of values per attribute.

Variants of a product can be created by adding the Attribute and values to a product in the variants tab in the product form.

Trying to create more than 1000 variants in a batch will result in a user error that will suggest having the variant creation mode as Dynamic. In which case it will only generate variants for a product as it gets added to a sale order.

Out of the box, Odoo allows to create 1000 variants at any given time. And hence I was able to create 1000 variants for the product. With just 4 attributes.

Once the variants are created, Odoo allows us to create more variants by adding more attributes or values to the product. To the same product when an additional 5th Attribute is introduced, It will create even more variants.

By increasing the values one at a time we could increase the number of variants for the product. This is a manual and tedious process.

There are limits to this as well, as Odoo will time out in 15 minutes. So, creating more and more variants will eventually result in a timeout.

Another important thing to keep in mind when creating variants (product.product) is every variant has its own form and even though they are considered variants they are stored in the database as individual records and having more variants would slow down the database.