I imagine this varies on a case by case basis. Since this is often not publicly disclosed, its difficult to know exactly. In my personal experience at a few firms, the seed money came with an MVP that had proven an ability to get users and provide value.

Your next question is how many users, I imagine. This varies by the product/service. Working B2B? You don't need thousands. Doing something consumer facing or social? You need a lot, and a hook. Or something no one else has done that catches someone's attention that has a lot of money.