We investigate spin-orbit torques (SOTs) in heterostructures with Pt1-xMnx alloy and CoFeB as a function of Mn composition (x) by using an extended harmonic Hall measurement. Slonczewski-like and field-like SOT efficiencies (xiSL andxiFL) show non-monotonic variation and a different trend with respect to x, and considerably largexiSL up to 0.21 is obtained at x=0.20. Compared to the x dependence of longitudinal resistivity, the Slonczewski-like SOT in the low x region is mainly attributed to an intrinsic spin-Hall mechanism, whereas a non-monotonic variation in the higher x region suggests the presence of additional factors. The present findings deliver useful clues to understand the physics behind SOT generation in antiferromagnetic heterostructures and offer a route to realize efficient devices.