So I have heard this alot, but how do orcs have so many number on their side? By my could they should be all dead, as even though the orcs can breed a great number of times, and the children reach adulthood at age 15 that still means one has to wait 15 yrs to get orc adults. This effectively means all the orcs in the world should be dead unless they have another method of building armies. I would also point out that official lore doesn't help with this as the orc life span is below 50 yrs, so it doesn't look like they can have long lasting soldiers. I didn't like the idea that they breed alot because A. That makes them somewhat human, and I would like to dehumanize them a bit and B. It would be weird if after crushing their numbers the party moves into their lair to find everyone "Building a new armie" Is there a better way to explan orc reproduction? 

Well, my first choice is to just not ever think about orc reproduction at all... but if you insist...

I think you're picturing one orc having one child every 15 years. That's not how it would work. The happy orc family could have a child at 15, then one at 16, 17, 18, etc. Maybe in your world they often have twins or triplets. But you don't even have to do that to get exponential growth. Maybe they have 5 years of baby-making before they become soldiers in the interest of keeping armies up.

Or if your goal is to dehumanize them, I'd go the LotR route and just have an evil wizard or something make them out of inorganic material (don't @ me about the actual detailed origins of Tolkien's uruk-hai, I'm just referring to how it looks in the movies).