So the trick is to extract the number as a field first. You can do this inline, but eventually you'll want to set up a field extraction for it.
To do the inline version, you'll want to rex out the field, like so:
sourcetype=mylogs | rex "\d+:\d+:\d+\s(?<fileCount>\d+)$" | where fileCount>=25
Now, if you take that same regex and use it to make a field extraction, then your search would simply be:
sourcetype=mylogs fileCount>=25
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