Logistics

i got some questions about logistics in modern militaries. suppose a unit of around 20000 troops(a division?), how many men of those 20000 would be logistics supply troops?
how many trucks or logistics vehichles would a division of 20000 have for supply?
and in today's battlefield, how would a division be supplied?
do supply trucks follow a few miles behind the tanks and rush up in case they need ammo? or do they pick a spot for refueling or do they just gather all they supplies at a depot?
we see in movies tanks reloading ammo and refueling during a lull in the fighting. but if a fight goes on for a long time, would the supply guys actually go up to the front and supply neccesities?

A lot... Basic artillery firebattery has 8-10 trucks (if its towed one) and Supply&HQ have similar level, perhaps even more trucks.
But a basic thumprole would be that every squad/group would need a truck or howler. If you count of tank and infantry wich would use their own equipment you still get lots of trucks.

Your question begs specifics that if rendered would make it less general and more specific. The methods of supplying troops have changed preciously little since World War II, although such a battle (division size) is unlikely in most modern scenarios because of its sheer size of mobility (i.e. Target.) Different countries tailor-make their offensive tactics based on any threat it perceives: the strength of the given nations force is wholly representative of that nations acuity to history, implementation and overall morale.

Lets take a modern example in 'Operation Iraqi Freedom' which was the latest battle to enforce those numbers. Structure was the key to (the overwhelmingly one-sided) victory and the Americans used a combination of counter-espionage, blitzkrieg, electrical warfare and air superiority. The detachments are randomly switched up and decoys are used to foil ambushes. The supply structure by the way is separated into battalion allotment. Ideally the Administrative (logistics) arm of the battalion is composed of a company -/+ depending on arrangement and is defended by a combat support element. The whole division hierarchy is maintained fluidly this way (in theory.)

The 250-300,000 coalition troops first of all had airlift supply routes secured from Kuwait and Turkey coupled with heavily armed convoys (most from Kuwait some from Saudi)

Supply chain management goes all the way back to the factory that makes the item needed by the warfighter and how that item is identified, shipped and provided to the warfighter in the field. Having a great batallion supply is useless if the top level supply management team is clueless, or the economy simply cannot churn out the things the forces in the field need to fight, or that economy makes too much of the wrong thing and not enough of the right thing because they lacked accurate information. It becomes a communications and organizational challenge that crosses all services and impinges on what the civlian economy can provide. The fighting forces have to be able to effectively communicate to the civilian acquisitions structure what their exact needs are so the civilian side can translate these needs into hardware and software. The old way was to simply throw material at the job. In Desert Storm for example fully 90% of the over 2000 shipping containers sent to the field had stuff that wasn't critical to completing the mission. The warfighter had what was needed, and a ton of extra that wasn't. We wasted a lot of effort shipping big heavy containers across hundreds of kilometers of desert when that effort could have been used more effectively elsewhere. None of these containers had accurate packing information, so all had to be unloaded and sorted to see what was available to the warfighter. Christmas in the desert, woo woo! More like a big cluster. In the latest Iraq invasion we shipped far less material to the field, many will tell you we cut this one a bit too close and I tend to agree, but we drastically reduced the amount of material used to conduct the actual invasion because we had a much better idea what each unit needed and what was in each container we shipped. In the future you will see FedEx quality service, with real time tracking of supplies, and what is called in the trade "visibility". The war fighter will be able to go on line and see exactly where the supply item they need is located in the supply system, identfy to the supply chain that warfighter's need, and move that item to the field with FedEx speed. In the past soldiers would order stuff, wait a while, think it was lost in space somewhere and order it again. This flooded the supply system with redundant requests and a flood of material only a portion of which was actually needed for mission completion. In the very near future the war fighter will be able to track his or her item in transit. When a container arrives in the field now, the supply side knows exactly what is inside and where each item is destined. No longer will there be a need to throw material at the situation and hope there is enough when the dookie hits the fan. This is in progress today with they US DoD.

From what I've heard, up to 1/3 of the total manpower of a division can be considered support personnel.