I live in calif and I have a large sum of ssi backpay. Social security has turned it over to the county medical services to take any monies I owe them. Is there alimit as to how much they can take?? Or can they take it all??

On what grounds do you owe SSI? Were you overpaid? Were you receiving benefits when you were not eligible? What is currently being garnished?

They can take it to the extent that you owe -

If this back pay is being paid to you and the county has provided you with services such as medical or other benefits, yes, they can take whatever monies that were outlayed on your behalf.