When looking for emails it is useful to view the oldest email first. You can do this by clicking on the email count and selecting 'Oldest'
Select any email list, where you have a count (e.g. 1 - 100 of 300).
Hover on the count to see a drop down showing the options Newest and Oldest.
This will sort the email in that list by date - newest first, or oldest first.
Where there is only a estimate (eg 1 -50 of many) it is an indication that Gmail has not yet counted all your email in the chosen list and the sort option will be greyed out.