I think the Republicans should easily gain 5 seats in the Senate.
Indiana
Missouri
Montana
North Dakota
West Virginia
...possibly others...
Ah if only. Indiana is shaping up to be a highly divisive primary. Missouri's candidates are shooting themselves in the foot. Tester has won twice and doesn't have a name opponent. Manchin has also won and doesn't have a really scary opponent. The only one where the Democrat is facing the best possible Republican opponent is North Dakota. On the other hand Nevada is probably a Republican loss and Arizona could be if the Republicans nominate the wrong candidate.
The GOP could pick up a seat or two, but they could just as easily lose a seat or two if they screw things up.
Florida, if Rick Scott runs...