Thousands can be managed easily if one does not go chasing transfer deals and consolidates at 1-3 registrars.
I have nearly 3000 now, 2600+ being with GD and it is piece of cake to manage. Basically just renew in time and respond to offers.
Planning to get to 10k names some time in 2021.
Even if you chase transfer deals it's possible to manage 10k domains by a one technically skilled person. Unfortunately most domainers I have seen on this forum and in other places are moderately computer illiterate. Most of them do domaining as a side business, are not full time domainers. So they do not have enough time to learn any computer skill.
This was one point when I said managing more than 200 domains is not an easy task.
Second point is;
You can't manage price updates, manual/direct sales and custom landing pages. You will eventually need to sell at marketplaces if you have more than 200 domains. Selling at marketplaces has advantages and disadvantages compared to direct sales.
More than 200 domains means a business model based on quantity. In my opinion it's more risky and offers low profit potential compared to less than 200 but higher priced, carefully picked domains.
Assume portfolio A has 10,000 and portfolio B has 200 domains.
Assume total worth of A = total worth B as a typical investor would have the same investment budget for domaining.
Problem starts here:
# domains in A / # domains in B = 50 = 10,000 /200
This means randomly chosen domain in B would have a 50 x price tag compared to a random domain in A.
Like, $500 vs $25,000 or $1,000 vs $50,000 or $ 3,000 vs $150,000 depending on total investment.
Questions:
1- Who will pay the renewal fees of extra 9,800 domain of the portfolio A ? 10,000 - 200 = 9,800.
2- Which is more likely? Selling a domain for $500 or $25,000 ?
If you say selling for $500 is more likely please think again about renewing extra 9,800 domains. 9,800 x $9 = $88k fees in renewal. Compare $500 in revenue vs $7,320 fee per month in renewals. You need to sell 50 x domains in portfolio A in the same time interval. In my opinion it's less likely. Because number of domain buyers is not too many regardless of the price.