What does "back build" mean? Do you mean he can duplicate the machine?
If he cannot duplicate the machine, and can only keep it in good repair, then his capacity to care for children is the bottleneck, along with capacity of the one machine to produce clones.
Presuming a batch of infants every nine months, humans are not able to be independent at all until the age of two or three (24-36 months) and not fully independent until the age of 5 (60 months) , which is going to mean about 30 infants he has to care for, feed clothe and dress, without any help, and possible 70 kids (63 months) before he has any fellow babysitters to help.
More likely, he is going to have to wait until the first batch is about four before he begins a second batch, and in general will have to start slow and ramp up. After about 20 years, he can probably run the cloning machine at full capacity with enough caregivers to not exhaust himself, so there is 80 years of full capacity. That is 960 months, about 107 rounds, of ten children each, for 1070 kids. In the first 20, batches are probably 3 years apart, maybe 70 kids, half of them at least 10 years old, so 35 kids to care and provide for 10 infants and 10 children. That sounds difficult but possible.
I'd say a population of 1100 to 1200.
If he can reproduce the machine, the population can grow much larger, possibly doubling every 10 years after the ramp-up period (still 20 years). Then you'd have 2^8 = 512x the initial population of about 70, roughly 35,000.
I hope I have provided enough detail for you to make your own estimates. I would spreadsheet this year by year, if I were you.