There are two limiting factors. First, the 100 variant limit per product, and second the total overall variant limit your Shopify plan allows.
For more information visit Does my store's variant limit come into play?
The Quantity Breaks (standalone) app is no longer available for new installations. If you are currently using Quantity Breaks (standalone), you will be able to continue using the app on a grandfathered plan.
Any new installs of the Quantity Break app will be redirected to Custom Pricing which has a Quantity Breaks plan available. For more information on Custom Pricing, please visit Custom Pricing: Information and Overview.