You get extra attacks based on your total Base Attack Bonus. Add the Base Attack Bonus from all your class levels (prestige or otherwise). You get two attacks at BAB +6, three attacks at BAB +11, four attacks at BAB +16. Your second attack is at BAB - 5, third attack is BAB - 10, fourth attack is BAB - 15.
Extra bonuses to hit (like Strength gives you a bonus to hit with melee attacks) are only added to your BAB when rolling to hit.