Is there a certain amount of blood needed to transfer hiv to another person trough a scratch?

Can't be done: A scratch / abrasion isn't an open wound. You're not going to get somebody else's blood into you that way. There's never been a case of transmission this way, just as the people who "got HIV from mosquitoes" actually didn't. Stop worry about this stuff. If you are obsessed with the idea that you may catch hiv, get with your physician -- unwanted thoughts can be treated ; you'll be more comfortable.

Amount of blood: That's a really tough question to answer. It really depends on viral load. Viral load refers to how many virus particles are present in the infected individual. This can vary dramatically depending on the time of the infection. At the time of initial infection the viral load can be very high so a small amount of blood may contain a very high concentration of viral particles. Get checked.