A lot will depend on the type of mortgage you are applying for.
For example, if you are applying for an FHA mortgage, the loan must conform to FHA requirements. FHA allows sellers to contribute up to 6% of the purchase price towards settlement costs.
For USDA Rural Development loans, the mortgage must adhere to USDA requirements. For USDA loans, there is no maximum limit to seller contributions. However, anything over 6% must be explained in writing.
For VA loans, the loan must adhere to VA requirements. The maximum seller contribution towards closing costs on a VA loan is 4% of the purchase price. Sellers can contribute up to another 2% of the purchase price to be used towards points to lower the buyer's mortgage interest rate.
If you are applying for a conventional mortgage, it must conform to Fannie Mae / Freddie Mac underwriting requirements. Fannie Mae / Freddie Mac allow a maximum 3% seller assist if you are putting less than 10% down. If you are putting between 10% and 25% down, the sellers can contribute up to 6% towards settlements costs. If you put more than 25% down, the sellers can contribute up to 9%.
All that being said, lenders can impose tighter restrictions (you sometimes here the term "lender overlays.").
Your mortgage loan originator should be able to explain these seller contribution limits to you.