Does the "sign up" give you monthly or annual tabular data?
The .CO Registry announced 2 million at the end of 2015 if I read that correctly. So if 200,000 per year (my guess, nothing more) would be 2.5 million by now? Plus new china sales if any.
https://blog.go.co/2016/01/25/by-the-numbers-2-million-co-d/
Here is a bit of info I dug up which has me scratching my head. You can buy a report for $27 per tld. But look at what they are saying, and think if this is the data source that some of these tld sites are using.
".CO 1.8 million ...
We separate our lists by country and not by individual zone files. This means that some lists will contain multiple zone files. For example, our .au list contains data from .com.au. .net.au, etc". So at this point I don't even know if .com.co + .co are lumped together by these tools.
https://viewdns.info/data/
What we need is data direct form the registry like NIC Mexico does.
I read that "Golden Gate Capital" took NeuStar private last year so I'm not sure if they will release the data anytime soon. It seems that the company is going through major changes at the moment. Being taken private like this looks like a good thing for them. Usually companies get reworked, sometimes split, then are able to come out stronger. They can't do that when traded publicly because the markets will crash them.
I've tried emailing Investor Relations, PR, etc asking for the data from 2010 by year/month in tabular form with annual if not monthly update. Please send out your own requests as reporters, investors, etc. But again they are private now so they might not respond.