Doves flying into a field are there to eat, and the sooner they can get on the ground the sooner they can get to dinner. In most situations, the ideal setup is to have enough shooters around the field to keep the birds in the air. If there aren't enough guns, the doves will land in a quiet spot. Decoys can help in either case.

If there are plenty of guns, a spread of decoys may convince the doves that particular spot is safe. If there aren't enough guns, the decoys can indicate that this is a pretty good spot out of all those available. It doesn't, however, mean that the birds will land among the decoys like waterfowl will. That can sometimes happen, but it's not something savvy hunters anticipate.

Instead, the fake birds frequently cause the real doves to change their flight path and wander over for a better look. If the decoys are properly placed, that's all it takes to bring them within range.

That is, essentially, the benefit of dove decoys in a field situation - they can often draw birds into the effective range of your position that might otherwise pass by out of range.
One effective way to do that is to determine what your maximum shooting range is - the furthest distance at which you feel comfortable taking a shot. Pace it off from your position and start placing decoys on the ground there. If the doves are over - or inside - those decoys, then you know they're in range. The remaining decoys can then be placed back towards your stand but shouldn't be put any closer than about 20 yards.
Veteran hunters have also found it is often more effective to put the decoys out in distinctly separated pairs, rather than groups. Doves on the ground often travel in pairs, and this is a more natural appearance than a large, compact group of decoys.
A dozen decoys on the ground is enough to be effective. If you have more, get them up off the ground.

Doves often land in a tree, or on a fence, to take a look around before they hit the ground. Elevated decoys are a natural-appearing situation that doves would expect to see.
One mistake to avoid, however, is placing any elevated decoys close to your stand. That can draw the birds' attention to you. Instead, take the time to walk 20 to 40 yards away, then get them as high as you can. Also make certain that they are visible from any angle and not screened by vegetation. If the clips on your decoys won't hold them on a skinny limb, a quick wrap of non-reflective plastic tape around the bottom of the clip normally keeps them in place.

If there is a "perfect spread" for dove decoys in a field, it would be a dozen or so on the ground between 20 and 40 yards in front of your stand, with another group placed 20 to 30 yards to either side of you in elevated positions.