The Republicans will be defending 18 seats and the Dems will be defending 16 seats, but unless there is another tsunami, each party will have 5 seats that look like they are at risk. (I assume a seat that was won by less than 5% in 2022 is at risk.)
Here are the Repub seats:
Murkowski AL
Moody FL
Budd NC
Ron Johnson WI
Mike Lee UT
I think only Budd in North Carolina and Johnson in Wisconsin are at serious risk, but with the directions those states are headed, I think they might both be underdogs in 2028.
Here are the Dem seats:
Kelly AZ
Warnock GA
Cortez Masto NV
Brown OH
Fetterman PA
I don't think Fetterman will be the nominee, but I think PA is pretty safe. In fact, the only seat that might be at risk is Brown in Ohio.
So, in a "normal" election, I think the seats in danger are 2 Repub seats and 1 Dem seat. Unless there is a big wave, not much should change in the Senate.
Of course, Trump has 2 years to get more unpopular, which could lead to another huge blue wave. Then fillibuster-proof 60 is possible.