The usual bonus offered for signing up is 50,000 points. Once in a while the offer is upped to 70,000. I waited for the higher bonus before I signed up and got the full 70,000 after meeting the spending requirement.
This would probably be a question best directed to Chase, but a call to the Marriott Rewards customer service couldn't hurt. I'd make a phone call rather than email as you'll probably get it resolved faster.
I had the same issue within the last week and sent a nice message to Chase customer service through the Chase Secure Message center asking for a bump from 50,000 to 70,000. They responded today notifying me that they would honor my request for a bump from 50,000 to 70,000 points and I would simply need to call once I hit the minimum spend and they would add an additional 20,000 points to my signup bonus.
Just ask nicely! :-)