All 50 conversations on this page are selected.(the number of emails may not be the same, it is not important), and next to it, the button
Select all 79 conversations in Primary(you will have a different number of messages). Click this button.
Inboxlabel, not just the first page. Instead of the
Select all ...button, you will see
All conversations in Primary are selected.
But if you want to select all incoming emails at all (not just from Inbox),
then in the list of labels on the left, go to
Next, do all the above items.
All Mail label includes all incoming mail except those in the