Our system normally allows for up to 10,000 discounts in total to be configured at a time in any one Seller's account; these could be any type of discounts, including a product-specific discount that marks down the per-unit price of that product by a flat amount. We do have a way to make your account auto-delete discounts as they expire (e.g., as soon as a Discount Code with Max. Use Count=1 is used for a completed order), so that would free up slots to add more discounts on-the-fly as needed; let us know if you'd like us to enable that for your account.
Development may be able to expand that limit for particular sellers, but that still leaves the matter of setting up and managing the codes. As you can see in Seller Admin > Cart Discount Settings, there really isn't any way to view or mass-edit multiple discounts at once in there; even if you create a discount by pasting in a list of codes, that actually creates a series of separate discounts, one for each code -- imagine having to scroll through a menu of 250,000 discounts to find any particular one you might want to view or edit. This would probably have to be a "set and forget" scenario where you won't need to view/edit any of your discounts after the codes have been set up.
Perhaps if you could explain what you mean to accomplish in more detail, we might suggest a better way of going about that. I'd suggest emailing support in reference to this forum thread, so we can refer any questions or special requests to Development as needed: