The US Senate has 100 members, and a majority requires 51 seats, especially because in the case of a tie, the Vice-President gets the tie-breaking vote. Democrats must theoretically get that 51 seats, but ever since Donald Trump has resettled into the white house, one democrat seems to have become a friend of republicans.
John Fetterman, the democratic senator from Pennsylvania, has voted with republicans repeatedly and has even shown his face at the midterm convention of the GOP. It seems likely that Democrats must thus flip not 4 but 5 seats this november to get to 52, and avoid being blocked by Fetterman, who can only be voted out of his seat in 2028.
You can check out which seats democrats seem poised to flip in our own polling map on World of Elections. Go to America, and you can view the Senate and House projections