Hi,
Firstly I would say 2 is the perfect number to have in any car, unless you have a large estate or MPV then 3 or 4 is going reduce the boot space for tents etc. It may reduce the costs slightly but having an extra person, let alone 3 in your car if its been a hot one will not make it pleasant.
Secondly, most sites do have numbered pitches, Houx Annexe for sure is all numbered this year and I think Blue and expo are as well. All 3 are very close to the track. (Numbered pitches have a size restriction on them, normally too small for a car + 4 tents, the link makes no mention if you have one pitch or 2)
I would say dont bother with motorracing international and do it of your own back. Get a cheap ferry, should be under £100 dover calais, find a camping pass on ebay or this site, (there will be more coming up over the next few weeks) try to avoid paying over face value!
Entrance tickets you can get on the gate and fuel is not really that bad now, factor in a bit for tolls etc
I think your figure of 180 - 200 pp is pretty optimistic tbh as the prices quoted on MRI dont seem to include entrance tickets do they?