I was in the same situation a year ago and I tested semen right before and right after because I was freezing a bunch for future use. Bottom line, in my case counts went down from a concentration of 198 million per ML (5ml ejaculate in the sample=400 million total motile count) to 44 million per ML (4ml in that ejaculate= 93.3 million total motile count). So it's a massive drop. Even while staying on FSH.
So the absence of HCG or reduction in dosage for a short time makes a big difference. At least in the context of being on low-dose TRT. However, 'Normal' is > 15M concentration and > 20M total motile count, so even with the break from HCG, I was still well above normal. Note, that's after a solid 6 months of higher dose Hcg and FSH, so testicles were pretty large and healthy.
It's a bit more complicated when you look at the full picture of course. I actually did an HCG shot a couple of days before my "after" test noted above-- so if I hadn't, my total volume would have gone down significantly (hcg definitely increases semen volume per ejaculate). Also, I was on HGH which helps counts. And have a bunch of other stuff in my regimen including danazol for shbg control and so on.