Dovecot and gmail support condstore, and any marketing professional will tell you that they alone cover 70% of the users. Whether you believe marketing math is another question, and whether 70% is a lot in your eyes is a third, and neither of those questions are appropriate for SO.
To check any particular server, do either openssl s_client -connect asdf.example.org:993 -crlf or telnet asdf.example.org 143 to connect, then a login asdf fdsa to log in and finally b capability to get the list of extensions. You will see condstore listed in the reply. Or not, as the case may be.
Edit: Due to @guettli's request I took a convenient set of email addresses now, and for each address I determined a few possible IMAP server names heuristically and tried to connect to port 993. I see >80% CONDSTORE now, if I count only the addresses for whom I could connect, and do a little bit of guesswork, such as assuming that all Dovecot servers are new enough to support CONDSTORE.
However, your address mix isn't like mine (which happens to be dominated by one particular freemail provider), and the way to count depends on your purpose. Do you want to count the big providers that have CONDSTORE, but whose users mostly use the web interface?
If you want to count servers rather than users, Shodan can help. Searches for imap generally and for the most popular server says 75% of servers have condstore and the remaining 25% may or may not. But of course that's a misleading way to count: most users are on a few freemail services and most of the rest are on just a few thousand other providers.
If you want to run a survey yourself, connect to ports 143 and 993 and send a capability and then CRLF. Condstore is supported if the response includes either condstore, qresync, anything to do with modseq, or if the server banner names Dovecot or Cyrus. (Some servers won't tell you whether they support condstore until you have logged in, so looking at the server name is a good proxy.)
