I am not sure number of user accounts but number of domain names registered there, at least at date of this Dofo blog article, was about 880 thousand, making them the 50th on list of registrars.
https://dofo.com/domain/registrars?page=2&page_size=25
Now many have large numbers of domains, so you would expect not as many accounts.
However, in a sense have a larger number of domain names 'under management' (I think Epik have in past used that term) because you could use their for-sale landers for domain names listed elsewhere.
Also, names on NameLiquidate that are registered elsewhere are on their site.
They may also have hosting or other service accounts without a domain name registered.
Recall that, as I understand it, the breach held some data for up to 10 years, so someone who had a single domain there years ago would in a sense be in the impacted number.
I have found it hard to follow all the claims, some of which have been written about by members of general press who don't understand certain aspects of whois, expiry, etc., but the number of people impacted would be expected to be higher than number of current users.
Bob