I assume you're talking about buying stock. When you do that, you're not actually giving money to the company unless it's an IPO; you're buying shares from other investors.

When companies raise money, they only raise a given amount. This is true from seed rounds through IPO and subsequent stock issuance. If people want to buy more shares than the company's issuing, some of them won't get shares (and raising the price of equity isn't always the correct answer, especially for an early stage startup).