You can't have more than the available sets (4, 16, 32 etc). If you are bothered by the blank unused ones you can simply zoom in to show only those occupied by your objects. Use the '+' and '-' buttons at the right bottom of the timelines. This is explained further in your programs 'Help' file available from the top menu bar or using [F1].
Regards,
Carl
ADDED: Thanks much asolei. I'd only ever used the radio buttons, even after knowing that several other option can be typed in eg aspect ratios IIRC.