tl;dr: There are currently 13 senate seats which could be flipped (plus one special case), 7 would go to the Republicans and 6 to the Democrats
Not including those states where an interim gubernatorial appointment is permitted but limited to 6 months or less (which accounts for four states where the governor and at least one senator are not from the same party), currently 5 states' governors have the ability and party preference to swing the senate to the Republicans, and 4 where there would be a swing to the Democrats.
To the Republicans:
||Both Jon Ossoff & Raphael Warnock
||Both Jeanne Shaheen & Maggie Hassan
|Richard Michael DeWine
To the Democrats
||Both Jerry Moran & Roger Marshall
||Both Mitch McConnell & Rand Paul
|Janet T. Mills
||Susan Collins (and technically, Angus King)
A note on Angus King, although an Independent, he does caucus with the Democratic party.
Arizona, Maryland, and North Carolina would fall under this, but the law states that replacement senators appointed by the governor must be of the same party as the senator who vacated the seat.
There are three states which would see interim gubernatorial appointments, where at least one senator from that state is currently not of the same party as the governor.
||max 11 weeks
||Bill Cassidy & John Kennedy
||max 160 days
||Elizabeth Warren & Ed Markey
||max 6 months
||Patrick Leahy & Bernie Sanders
The only state where there is a disparity between one or both senators and the governor where the governor can not appoint, even temporarily, a replacement senator would be Wisconsin (Ron Johnson).
Sources: Ballotpedia & Wikipedia