Here's the 270 to win Senate map.
https://www.270towin.com/2020-senate-election/
My guess in order of risk of party switching:
Tier 1 Switch (Most likely):
Alabama senator Doug Jones-D
Colorado senator Cory Gardner-R
Tier 2 Switch (Likely)
Arizona senator Martha McSally-R
Tier 3 Switch (Toss-Up; and in order of most likely to switch)
North Carolina senator Thom Tillis-R
Maine senator Susan Collins-R
Iowa senator Joni Ernst-R
Montana senator Steve Daines-R
If you put me on the spot; I'd say that Democrats win Colorado and Arizona Senate seats and lose Alabama. Net +1 for D.
Collins survives; it's Maine, they are weird up there. And the other states' senators likely survive in even a Biden + 5 victory, although North Carolina is the next likely to fall. I don't see Trump losing Iowa or Montana and ticket splitting is pretty rare nowadays.
The scenario where Dems get the majority involve Biden+6 or more and both Tills and Collins fall. Then its 50-50 and Biden's VP is the tie-breaking vote.