IcyCool said:
Well, lets see. According to the CR system, a single 1st-level barbarian is a CR 1. Two of those is a CR 3, four of them is CR 5, 8 of them is CR 7 and 16 of them is CR 9. CR 9-10 is a very challenging encounter for a 6th level party. So, my suggestion (since you require it to be 15 barbarians (and assuming you want them to be the barbarian class):
14 1st level Barbarians armed with an assortment of two-handed weapons (no polearms and such, stick with longspears, great axes, great swords, and great clubs).
1 3rd level Barbarian as the leader, with similar kit.
The CR system totally falls apart for large numbers of wimpy opponents.
A few weeks back, I ran an encounter with 47 2HD Hobgoblins, a 3rd level Adept, a 6th level Adept and a Rogue 1/Fighter 4. Although a long fight (and the 50 Hobgoblins took 4 rounds for all of them to arrive), the PCs won easily (3 7th level, 3 6th level, 1 5th level NPC cohort and 1 4th level NPC cohort) and this was with Hobgoblins grappling known spell casters.
5 6th level PCs should mop up the floor with 14 1st Barbarians and 1 3rd Barbarian.
If the OP wants a tough fight, I would up Frank's example to:
4 CR1 Barbarian 1
4 CR2 Barbarian 1, warrior 2
4 CR3 Barbarian 2, warrior 2
2 CR4 Barbarian 3, warrior 2
1 CR5 Barbarian 4, warrior 2 leader
That's 3 opponents for each of the 5 PCs with an average CR of opponent under 3. Course, it depends on the PC mix. Most PCs in my game can cast spells of some sort (we have one fighter who cannot). Spells are the great equalizer, so if the PCs include 3 or more spell casting capable characters, then this should be a tough but not overwhelming fight. With fewer spell casters or with some inexperienced players, the CR should probably be lowered.