I have no idea whatsoever. I think some of the orcs that Saruman drew into his service come from the Misty Mountains. I remember reading across a quote in the Return of the King (I can't remember where) which an orc said he claimed to be in the host of orcs that battled the Last Alliance. This could possibly imply that orcs could be immortal, since one of the theories of origins of orcs are that they were corrupted, tortured and mutilated forms of elves. And first bred by Morgoth. When the Battle of the Last Alliance ended it resulted in the entire orc force destroyed; or at least most of them. I think some of them fled into the misty Mountains and hence some became the Goblins you see in 'The Hobbit' or the Moria orcs. Saruman might have lured some orcs to his service through his powerful voice. I don't know how Saruman or Sauron could have done this.
I don't know about orcs reproducing sexually, but Morgoth, the creator of orcs, could not create life, only mockeries of it. I don't know what this means, but it is a hint as to how orcs can reproduce. Based on what I read of Tolkien, there is not even one reference to female orcs but I am just going to say that orcs reproduce sexually and that it is a big factor as to how dark lord wannabes manage to find orcs.
I am eager to find out too.
One does not simply have an orc army. Its army is more than just orcs, but of men, trolls, balrogs, dragons, and other creatures unknown. Not with 100000 orcs could you create an army. It is folly. 

Just KIDDING
Glorthelion