The total generic ballot for recent midterms:
2022 - GOP 54.2M, DEM 51.2M
2018 - DEM 60.6M, GOP 50.9M
2014 - GOP 40.1M, DEM 35.6M
2010 - GOP 44.8M, DEM 39.0M
2006 - DEM 42.3M, GOP 35.9M
Turnout models tend to estimate that we will see higher raw turnout than 2018, with turnout hovering around 130 Million voters in lofty estimates. If the GOP can find 60 Million total voters (which would be ~77% of Trump's 2024 total), they avoid a wipeout in even the doomsday scenario of high turnout. If the more "feasible" estimate of a 123 Million two-party voter turnout, getting 60 Million voters would allow for a reverse 2022 scenario. That's the magic number.
People forget the GOP actually had respectable turnout in 2025. In fact, had they gotten that level in 2021, they would have actually beaten Phil Murphy. Issue is if the other side is more energized. The overall gap today is the real question, though I must say that an enthusiastic vote counts just as much as an unenthusiastic one.