If all my shots were going to be in thick brush, I would be tempted to shoot 1 buck. There are 24 pellets in a 3-inch 12 gauge load and with an IC choke you get a nice-sized pattern at 25 yards.
Buckshot size and speed doesn't matter as much as buckshot quality. Premium, buffered loads of hard pellets are worth the price you pay for them for shooting at longer ranges (40 yards). Modified choke, by the way, shoots very good patterns with many different loads.
Buckshot patterns fall apart quickly. The load that is a sure killer at 40 yards may be a crippler at 50.
For really long range shooting, Federal's 00 buck loaded in its Flitecontrol wad is hard to beat for tight patterns. Check out the video above. It's amazing how tight a pattern this load shoots into the zombie target's chest at 50 yards. If Federal made their heavier than lead "Heavyweight" pellets in buck sizes and loaded them with a Flitecontrol wad, who knows how far away you could shoot deer with them.