I actually think my numbers are a fair bit high, but wanted to toss out a number to start with. Considering the devastation of the Hegemony industry and the losses incurred in its reconquest I could see the numbers being decreased to as much as half what I suggested, but there's a huge amount of crunch there and large amounts of cached equipment could go a long way to making up those losses.
Most clans seem to have around 750 ASF (I, in another thread, calculated the Scorpions to have ~800 ASF in 3059 and they are noted as being ASF-lite, iirc). So that's ~13,000ish ASF among the clans. In the few clans we actually see TO&Es second line forces seem to make very heavy use of SLDF designs (granted those products were put out before real 2nd line designs were introduced). So you could see as many as 5,000 SLDF-era ASF in active service - and that's before we start talking about losses over 250 years, which could suggest there shouldn't be any SLDF designs remaining (my suspicions of ~12.33% yearly loss rate for ASF is a massive amount over such a period of time).
I'm not sure what the actual numbers should be, but ~4200 seem a little high, though the difference between serviceable and broken may produce numbers closer to what I would expect.