mmadsen said:
Do the goblins have to sneak up on the adventurers? Or can they let the adventurers walk into an ambush?
There are lots of ways to get large numbers of fodder monsters close to the PCs (and there are lots of ways for the PCs to counter these tactics...and they should be allowed to do so).
In rough terrain where visibility is limitted, set up an ambush. A small number of skirmishers hide near the path of the PCs (a larger force would be discovered too easily). A much larger force is set up nearby. It is divided into 2-4 components that can come at the PCs from several directions. Upon engaging the PCs, the skirmishers give signals that communicate how the PCs have set up (where their strong point is). Then the main force begins to move in. First they come against the party's weak side and then after a few rounds, the rest comes against the strong side (presumably while it is shifting and in a state of confusion). Ideally, only some of the monsters will try to close at first. The rest will rain down arrows or other missile weapons from cover and force the PCs to counter attack one side of the ambush. When they do, the other side will charge and try to attack from behind. If the ambushers can keep forcing the PCs to change their plan of action and shift their concentration, they can really do some damage.
A couple tips about carrying out an attack:
Vary the pace of the attack: Don't have the waves coming in predictable intervals.
Don't attack all at once. Keep a portion of your force unengaged (and even hidden) so that you can use it to react to unforeseen situations.
Play to your advantages and against the enemy's advantages...if you can move quicker, move a lot. If you are better at close quarters, close the distance as fast as you can. If they are able to launch mass damage attacks (like fireballs) spread out.
Time the attack so it occurs when the enemy is at his weakest or is off guard (like first thing in the morning before the wizard has finished preping his spells and the fighter is in the middle of donning his armor).
Use diversions. Does the party often split up? Does it send the rogue and ranger ahead to scout out a strange noise? Use it against them.
To be fair, some of these tactics require the 'horde' to have knowledge of the party. If they don't it would be unfair to use some of these tactics (however, some of them it could be argued are reasonable guesses). Of course, you could have the monsters recon the party with a few hit and run attacks to learn their tactics.