When you search in Gmail it appears that any search over the number displayed (e.g. 50) will show "50 of many" instead of the total number.
There is an option to go to most recent messages, but the option to go to the oldest messages is disabled (visible but greyed out).
I tried smaller searches and it seems that nothing over 50 will show the total number.
I found an answer to my own question when searching on the subject. (See below)
P.S. Why would I care how many messages there are in a search result? I want to figure out if messages are disappearing with a clear statistic. Some messages seem to have disappeared with no clear cause.