Clarification 

When you say Piano tuners, I’m assuming you mean full time tuners only

Here’s how I’ll be approaching the problem, I’ll calculate number of Pianos in SF and from there will back calculate how big the market is for piano tuners.

Does this include pianos at home and say hotels, theatres etc? Yes it includes all such pianos

Context 

Pianos are an expensive instrument and take up a lot of space ( 5 ft long and 2 feet wide, some pianos are even bigger)

Number of households in SF = 1 million/3 = 333k

We can look at household sizes in SF to create cohorts of population and individually map piano penetration to each.

total tunes in a year = ~187k

Assuming each tune takes 1 hour and can be done at home(since pianos are difficult to transport) 1 hour to commute and 60% efficiency with 9 hour work days 5 days a week, there should be 265 tuners in SF for private pianos

I’ll assume that there another 25% pianos in hotels bars etc (mostly expensive places have pianos), with that there should be 361 tuners in SF

Clarifying questions and assumptions:

Piano Tuners – These are people who tune pianos so that the piano produces the right notes. They are called on-demand by individuals with pianos or by places like music schools etc.

Assuming SF population ? 1M

Equation:

# of piano tuners in SF = # of users with pianos x avg.piano tuner request frequency

# of users with piano in SF = # of households in SF with piano + # of individuals with pianos

SF is primarily occupied by singles or younger couples who don’t necessarily have a traditional household, Thus assuming 60% of SF is individuals and 40% are households.

400 individuals are part of household ? 400K/3 ? 133K households

600k individuals

Also, pianos are expensive to buy and maintain, so I am assuming only 50% of households and 25% individuals can actually potentially afford it. Assuming the household has a higher chance of buying it since there are usually 2 or more sources of income.

Potential SF TAM for Pianos

133K x 0.50 ? 65K

600K/4 -> 150K

Assuming, 20% of this TAM actually has pianos in San Franciso ? 0.2 x (150K+65K)

43K pianos are there in San Francisco.

In general, pianos would require tuning atleast once a quarter and a tuner can tune about 3 pianos per day.

Thus the number of piano tuners needed to meet this demand

43K/(3×90) = 160 piano tuners are in San Franciso