solveSystem(...,StartSystems=>true,numThreads=>ZZ)
Use numThreads=>4 to run the path trackers with 4 threads.
The source of this document is in /build/macaulay2-88fgJW/macaulay2-1.25.11+ds/M2/Macaulay2/packages/PHCpack/PHCpackDoc.m2:741:0.