For competitive seats the democrats will be defending Michigan and Georgia (Ossoff) and the republicans will only be defending Maine. With the democrats likely only having 47 senate seats after 2024 election it's hard to see why this shouldn't be lower. Especially because the democrats need 51 seats in 2026 since Trump will be in office.

https://en.wikipedia.org/wiki/2026_United_States_Senate_elections#Georgia

@DanielBets Joni Ernst won by 6 points in 2020, Thom Tillis is retiring in competitive NC, Vivek might make Ohio competitive, Beto came within 2 points of beating Ted Cruz in 2018, Alaska is a ~10% shot for Dems etc That is 6 plausible seats (not likely, but still). In the right environment there are no non-competitive seats