Verlander, Cole, Morton, Keuchel, and McCullers have combined to allow just 83 runs in their 48 starts so far, pretty crazy. Here is a whole article that goes into the question asked in the title:
" The fewest runs allowed by a rotation in the modern era (since 1920, non-strike years) are 342, by the 1967 White Sox, who pitched right in the middle of the historically low-offense late-60s. If the Astros were to maintain this pace -- easier said than done, of course, and perhaps impossible -- they would allow about 300 runs. They'd shatter the record."
https://www.mlb.com/news/astros-rotation-could-rank-among-best-ever/c-277542622
" The fewest runs allowed by a rotation in the modern era (since 1920, non-strike years) are 342, by the 1967 White Sox, who pitched right in the middle of the historically low-offense late-60s. If the Astros were to maintain this pace -- easier said than done, of course, and perhaps impossible -- they would allow about 300 runs. They'd shatter the record."
https://www.mlb.com/news/astros-rotation-could-rank-among-best-ever/c-277542622