How much a person earns from Social Security depends on the year, current age and full retirement age (based on year of birth). As of 2014, you are able to earn up to $15,480 before deductions if you are collecting early benefits (available at age 62).
If you are collecting early benefits and are earning more than $15,480 for the entire year, your benefits will receive a deduction of $1 for every $2 you earn above the limit.
Here is an example from the Social Security Administration website. If you work and earn $23,480 ($8,000 over the $15,480 limit) in 2014, then your Social Security benefits would be reduced by $4,000 ($1 for every $2 you earned over the limit). However, you would still receive $5,600 of your $9,600 in benefits for 2014 ($9,600 - $4,000 = $5,600). If you will reach full retirement age in 2014, the limit on your earnings, for the months before full retirement age, is $41,400. If you earn more than $41,400 in the months before you reach your full retirement age, then your benefits will have a deduction of $1 for every $3 earned over the limit.
Though you may receive deductions for working while collecting, you may be able to increase your lifetime earnings average, which could increase your future benefits at full retirement age. There is a retirement earnings calculator available at SSA.gov.