Can admin limit the products to a seller, say 10 or 20 or 100 products?

The admin cannot put a limit on the number of products for each seller. But you will have the option of approving or disapproving a product once the seller uploads it. It will be displayed on your store when you approve and publish the product. You can also see the "pending approval", "approved", "rejected" products in your admin panel.

For more information about the approval flow of sellers products, please refer to the following help topic:
https://help.storehippo.com/topic/approval-flow-for-seller-products