If you create a course with more than 20 controls then P-cards cannot be used, as they only store 20. People sometimes accidentally punch an extra control (not on their course) so 19 is a safer maximum.
If you want to use more controls and are prepared to ensure P-cards are not used (by offering load SI-sticks maybe) then a sensible maximum is 29, as SI5 and SI8 sticks only hold 30 controls. Unlike P-cards, at least these sticks still record the Finish time even if all control slots are full. Our loaner sticks are Si5, and some people still have them as well (6 digit numbers.) Many people have SI8 - these are numbered 2nnnnnn.
If sticks overflow then the early controls are not overwritten - instead the extra ones are ignored. In this case a valid run can be awarded if the extra controls are physically punched on the map.
fyi: "Comcard Up" hold 30, SI9 hold 50, and all others hold 128 controls.