If a corpse cart with a Necro joins a unit, does it count as just one model for rank bonus, or the amout of models it displaces like calvary?

the number it displaces. There use to be an example of it in the old book, but I couldn't find it in 7th edition.

it's based on unit strength I believe.

should be number displaced, the reason I say this is the other unit that is simlar that is used is the bell and it counts for the number it displaces

neither of those 2 examples are chariots both the corpse cart and the bell act as chariots, and according to the rules for mounted they count towards rank for how many they displace I do belive, no clue on harry and I would say he doesnt matter since there is really no point when I will fight him

cool, just trying to make sure I understand all this madness!

Bruce is correct... well met

something to remember is that if you are larger than the base size of the unit you join, you get no look-out-sir. There are exceptions in the individual army codexs, but they are rare. Not a big deal unless you fight something with artillary like dwarves or empire, but something to keep in mind. It is why I am modeling Tyrion on a standard cav base instead of the larger one he comes with. His horse no longer counts a monster, GW jusr doesn't want to change the model. At least that is what they told me .