But where does Social Security get that number from? How is it figured out? Will the monthly allocation of funds equal what you were earning while working?

The Social Security Administration obtains your IRS records to determine if you are eligible.  Over the course of your employment history, as long as you paid Social Security taxes, you have a chance of obtaining disability benefits if your impairments prevent you from working.

Check out this detailed piece on how much money you might be entitled to if found disabled by the Social Security Administration.