For elderly dependent who is disabled, the gross income for the tax year must be less than $4,050.

My mom receives about $27000 between SS and pension. She doesn't pay any taxes either. But that's a different question than the one the OP is asking. The OP is asking how much of SS is taxable for purposes of the $4050 income limit that applies for purposes of determining whether a parent can be claimed as a dependent.

My Mom received about 18k in SS plus 2k in pension. She paid no taxes. IRS told her to stop filing.

That is very variable. It depends on how much other income the individual has.