Data for the past 10 years is a bit inaccurate. There are actually less dogs than this in Japan because Shikoku born overseas are included in this number. In 2020 for example, around 10 percent of registered pups were born outside Japan.
Best case scenario, the average Shikoku lifespan is 15 years. That would give us a population of 4775 Shikoku in Japan. If we divide that in half, there are roughly 2387 males and 2387 females of various ages in our imaginary population.
Assuming that in our imaginary population, males are fertile and used as stud till age 10, that gives us 1428 males that could possibly be used to further the population. Since Japan has just revised its law to only let females under 6 years old be bred, but under 7 years old if the bitch has had less than 6 litters, we will tally bitches up to 7 years old which gives us approximately 997 females.
While it would seem we have a viable population of around 2425 animals, the majority of these are not owned by breeders. A number that I've heard bandied about is that between 20-25 percent of our dogs end up kept for breeding programs here in Japan. If we assume 25 percent of females, that is 249 breeding females in the whole country. While we could assume 357 males at 25 percent, the true number is massively lower since the number of males kept by kennels is far lower than females. Most kennels will have 1 or 2 males if any, case in point my house where there are 6 females and 1 male. Here in the greater Kanto region there are probably around 10 regularly used studs now.
While we have 249 females in our imaginary gene pool, we only had 264 puppies born in 2021. Shikoku litters are generally between 3-5 pups, so let's use 4 as our median. That gives us 66 litters. Given that since 2011 the breed has been wandering between 200-300 pups/year, we can safely assume that we have around 50-75 actively bred females in any given year in the whole country (this is assuming 1 litter/female).
My little exercise in mathematics is making a lot of assumption, but however we play it, our breeding population is extremely small. I believe the next step in our preservation effort must be to find out exactly how large our breeding population actually is, and where these dogs are. Only then can we try to make plans to best use the gene pool still available to us.