It depends entirely on how many waypoints you have in total. And, on the wpt loading method you choose.
I and some others use a system that confirms CDU2 NAV operational.
Wypts 1-9 from card 1 are entered into both CDU's. when 8-9 is being tracked CDU1 is switched to 0-9. CDU2 is loaded with the next set of wypts, switched to 0-1 and then NAV. CDU1 is now loaded and then switched to 0-1 and then NAV. This system requires wypt9 (or last wypt e.g.7) to also be wypt1 of the next load card. It may seem a bit pedantic but it works and gives the crew time to check everything without rushing. Remember in the real world when this particular INS system was/is used every entry into the CDU was cross checked by every member of the cockpit crew. If you remember the incident when a Korean jet was shot down they did everything correctly except they didn't link the INS to the NAV autopilot. So the INS wasn't steering the a/c as it should have been.
The other thing to note is the distance between wypts. I for example have card 2 of 3 that finishes with wypt 7. This is because on that particular flightplan there are only 14nm between wypt7 and the theoretical wypt9 Thus wypt7 appears (as above) as wypt1 in card3 and what would have been wypts 8&9 are wypts2&3 in card 3.
Here is an example of card1 & 2:-
Code:;ADEU Card #1 for HLLB to LIRF
1 N 32*48.3 E 019*15.6 ; RZAAM
2 N 33*32.4 E 018*43.0 ; FARUJ
3 N 34*20.0 E 018*08.0 ; OLMAX
4 N 35*12.3 E 017*28.1 ; BEKNI
5 N 35*20.2 E 017*22.0 ; LASGO
6 N 35*52.9 E 016*56.5 ; EVIRA
7 N 36*30.0 E 016*27.1 ; ASKOT
8 N 36*51.5 E 016*09.7 ; ERNAM
9 N 37*34.5 E 015*51.6 ; PIMOR
;ADEU Card #2 for HLLB to LIRF
1 N 37*34.5 E 015*51.6 ; PIMOR
2 N 38*04.6 E 015*38.7 ; RCA
3 N 38*32.9 E 015*24.4 ; PIGER
4 N 38*48.0 E 015*16.6 ; VULKA
5 N 39*05.1 E 015*07.8 ; SOBRI
6 N 39*48.5 E 014*45.1 ; AGNIS
7 N 40*00.9 E 014*38.5 ; VARAL
8 N 40*35.0 E 014*20.1 ; SOR
9 N 41*32.5 E 012*55.1 ; LAT
vololiberista