It's very simple, actually.
In member.php, search for $thisstylename to get you near the correct area (exact like is 739 in my modified file in 2.2.6).
Replace:
PHP Code:
$stylesets=$DB_site->query("SELECT * FROM style WHERE userselect=1 ORDER BY title");
With this query from FireFly (line break added):
PHP Code:
$stylesets=$DB_site->query("SELECT style.title, user.styleid, COUNT(*) AS count FROM user
LEFT JOIN style USING (styleid) WHERE style.styleid IS NOT NULL AND style.userselect=1 GROUP BY user.styleid");
A few lines down, right after this:
PHP Code:
$thisstylename = $thisset['title'];
Add this:
PHP Code:
$thisstylecount = $thisset['count'];
Then change the template modifyoptions_styleset to look something like:
PHP Code:
<option value="$thisid" $selected> $thisstylename ($thisstylecount users)</option>
And that should do it. At least, that worked in my forums (I just tried it).