Anyone born in 1929 or later needs 10 years of work (40 credits) to be eligible for retirement benefits.
The maximum credits that can be earned per year is 4. Credits are earned based on wages as an employee or self-employment income. The amount of earning required per year per credit was $1,510 Therefor earning of $6,040 or more would equate into 4 credits. The earning requirements are currently indexed for inflation.
For more information on past earnings required for credit see the attached chart.