This is a duplicate of this market but modified to be cumulative-set instead of single-answer: 

https://manifold.markets/Panfilo/how-many-seats-will-the-democrats-h

Resolution will be based on how that market is resolved. 

This is the description of that market at the time of market creation: "I will add new options if the odds credibly swing either much higher or much lower than the currently expected median. Feel free to ask clarifying questions. By default, this market will resolve on January 4th, 2027. If there is an edge case in which it is unclear how many Dems are in the new senate, I will delay resolution. Independents will count towards the party they caucus with. Independents who do not caucus with the Dems will not count for them, even if they usually vote with them."

Regardless of what happens, this market will resolve as that market does but in a cumulative fashion (i.e. if the other market resolves 50, then this market will resolve YES for all options up to 50, and no from 51 onwards).