The vast majority of drawing tablet pens have two buttons.
Though some consumer Wacom models have only a single button.
A few pens have three buttons - for example a Wacom Pro Pen 3
These are the default settings for Wacom pens with two buttons.
lower button (the one closer to the nib) set to perform a mouse right-click
upper button (the one further away from the nib) set to perform a mouse left double-click
The actions taken by the buttons can be customized. Usualy to perform either a mouse action (left click, right click, a keypress, a system action, or to do nothing)
Other things people configure their buttons to do:
MOUSE DOUBLE CLICK - because same app use a double click
CTRL-Z keypress to perform UNDO
SPACE keypress - in many apps, SPACE + dragging pen will pan the drawing canvas
CTRL+SPACE keypress - in many apps, this will zoom into and out of the canvas (Krita, Photoshop, Illustrator)
NO ACTION - sometimes people accidentally press the buttons too oftern, so they set the button to do nothing.