There is no limit to import products.
As per the new Shopify API rate limit guidelines: after a store has 50K variants/products, no more than 1,000 new variants/products can be added via API (apps) per day. Hence, the app is not able to add new products. Shopify enforces rate limiting in order to prevent abuse of the platform.
In most cases, you will have to wait for a few hours before you can restart adding products.