This doesn't track my experience. I have Ewing's Sarcoma, and my first regimen involved 5 agents: Doxorubicin, Vincristine, Cytoxan, Ifofsfomide, and Etoposide. This [1] appears to be the same plan I used. When that failed, I had a second regimen with Vincristine, Irinotecan, and Temozolomide. After that failed, I had Irinotecan and Trabectidin, followed by Doxil, followed by, Cytoxan and Topotecan, followed by high dose Ifofsfomide. So only two of the treatments I received were single agent, but maybe it depends on the cancer.
[1] https://www.cancerresearchuk.org/about-cancer/treatment/drug...