100. If you get an error message while attempting to add variants to a product, it means that you've exceeded the limit of 100 variants per product. This limit cannot be raised for any account or plan.

You can only have a maximum of 3 options per product, for example Size, Color and Style. Note that these options can differ from product to product. This limit of 3 cannot be raised for any Shopify account or plan.

Resource: Variant Limits (Shopify Help Center)