In 2007, you receive one credit for each $1,000 in earnings you have, with a maximum of four credits per year ($4,000 for the maximum four credits). The amount of earnings needed to obtain one credit increases each year. Most people need 40 credits (which takes 10 years to mount up) to be eligible for benefits, although disability or survivor benefits may require fewer credits.
Even though you might obtain more credits than you need to be eligible for Social Security benefits during your working lifetime, the extra credits do not increase your benefit. What does increase your benefits is the total income you earned and made donations on.