In 5th edition D&D, it's perfectly within the rules as written to create a Clone Army - an infinite number of simulacra of yourself.
Creating a Clone Army
- Learn the spells Wish and Simulacrum.
- Get hold of the materials to cast Simulacrum on yourself once (enough snow or ice to make an identically sized copy of yourself; a strand of your hair, fingernail or similar; and 1500gp worth of powdered ruby). These can easily be obtained with the Wish spell since it can spontaneously create items worth up to 25,000 gp - just remember to rest afterwards to regain your use of Wish if you obtain the materials this way. (Never do this. Casting wish for anything except duplicating a spell has a 1/3 chance of burning the ability out of you forever, that's why you have your simulacra cast wish for you. Obtain the ingredients some other way.)
- Create a simulacrum of yourself. This is an exact copy of yourself under your control. It has half of your health, is made from ice and knows every spell that you do - including Wish. It can't regain spell slots so it can only cast Wish once, but that's all you need.
- One of the uses of Wish is to duplicate the effects of any spell at all of level 8 and below, but with no material requirements and no cast time aside from that required to use Wish - one action. Order your simulacrum to use its Wish to duplicate the effects of Simulacrum with you as its target. This will create another simulacrum of you, which also has the spell Wish learned and available to cast.
- Repeat the above order with your new simulacrum. This will create another simulacrum of you, which again has Wish. Rinse and repeat.
Each simulacrum takes 6 seconds to produce (the length of 1 action) and is a perfect clone of you in terms of stats, skills etc. except with half your health. This means they have access to every spell and spell slot that you do at the time of Wish casting. Simulacra can't replenish spell slots, so they can't regain the ninth level slot used to continue the chain of clones and once they have used all of their spell slots they can no longer cast spells. However, since they have access to every spell slot you possess and require just six seconds to replace, any opponent can easily be defeated by an army of high-level casters. Given 24 hours to prepare, you can produce 14,400 clones capable of casting, for example, 14,400 Disintegrates at once for 144000d6+576000 damage in one round. The Tarrasque ain't got shit.