If you put more buttons on it than it can accommodate, when you click
on the double-arrow at the right end, a second bar appears with the
rest of them. But you can't make them be permanently visible.
A way to deal with this would be to move the ones you use more often
toward the left end, so they're always visible. (In the Customize QAT
panel, select a command and use the up and down arrows on the right to
change its position.)