SF (not counting San Jose) rarely had more than two Top 40s at any one time from the 50s through the 70s.
KOBY (1550) had the format to itself beginning in 1956.
KYA (1260) flipped to Top 40 in 1958.
KEWB (910) made it three in July of 1959. KOBY dropped out in September of 1960.
That year and two months, from KEWB's flip to KOBY's departure, was the longest stretch that San Francisco had three Top 40s in the 50s-70s time period.
KNBR (680) flirted with it for a few weeks in 1965 but went back to MOR quickly.
KFRC (610) flipped to Top 40 in February of 1966, but KEWB dropped out in June of that year, changing format to MOR and calls to KNEW.
There wasn't an in-city Top 40 competitor to KFRC and KYA until 1973, when KSFX fipped from album rock to Top 40. That lasted less than a year before KSFX went disco.
And the next in-city Top 40 competitor was KCBS-FM in 1979, but it was automated and never really got traction.
After that, it's hard to track. The eighties, especially the early 80s, were filled with stations calling themselves Adult Contemporary that sounded and acted like Top 40s (KYUU and KIOI chief among them).