Tab selects the next note. Tab selects the previous note. Tab or Tab adds the next/previous note to the selection.
While in mouse object mode, you can click on a note to select it. Once you have selected one note, Left-click on another to select all notes between them. To add or remove a note to/from the selection, click Left. You can also click and drag outside of a note to rubberband select a series of notes.
Three different selection operations are possible if you switch to mouse range mode:
If Edit > Preferences > MIDI > Sound MIDI notes as they are selected is enabled, Ardour will send a pair of NoteOn/NoteOff messages through the track, which will typically allow you to hear each note as it is selected.