After years of training your mind, body and Ki as a monk, you have begun the path of an ancient and mythical form of martial arts. The Way of the Split Self focuses on de-localising one's mind and Ki from their corporeal form, allowing for stronger manipulation and control over it. This allows one to draw upon their inner strength and Ki, removing it from their body and realising it in the form of another body, splitting themself into several selves.
You divide much like a cell, facilitating a strong defense through confusion and misdirection, good battlefield control by being in several places at once, or the disabling of a single target with a tirade of fists.

While you have replicated yourself, your mind and Ki can move fluidly between your selves, spreading further and further out from your physical form as your mastery of the Split Self progresses.

Split Self

Starting 3rd level, your mastery of the split self begins with the ability to channel your ki and focus it into the form of a "replica" of yourself.
You spend 2 Ki points and 1 hit die as an action to create a replica with hit points equal to your hit die + your proficiency bonus + your WIS modifier. It has the same AC and movement as you, and splits off from you and appears in the space next to you. This replica is wearing the robes that you are wearing but without any magical effects that they may have, and has no weapon.
It persists until it is reduced to 0 hit points or until you (the prime) are incapacitated, in these cases it disintegrates into a glowing dust that fades after a few seconds.

The Ki that you personified in the replica can also be re-absorbed if it is still intact; you can spend an action to add the remaining hit points of the replica onto your own, cannibalising it. Alternatively, you can spend a bonus action to re-absorb the Ki but keep it separate, allowing you to use an action to re-form the replica with the same HP it had before without the Ki or hit die cost.

You perceive and experience everything that your replicas do as you are effectively a hive mind, but your ability to keep control of your multiple selves diminishes if they are too far from you.

Your replica has the same initiative as you, but takes its turn after you do. It can take any action that you can, as well as being able to draw upon your Ki to use features such as Ki-empowered or stunning strikes.
Your replica cannot be a distance further away from you than your movement speed, otherwise they fall unconscious. In this case you can re-absorb them as above, but only for half of their remaining hit points.

While your replica is within your concentration radius:
You can mentally command them, allowing you to move them on your turn while they remain within the radius,
They can use any reaction that you can.
When you and your replica both make a save, you (the prime) take the better roll and your replica takes the worse one.

Additionally, your Flurry of Blows ability can no longer be used to make 2 attacks yourself, but allows you as well as any replicas within your concentration radius to make an unarmed strike as a bonus action.

Fluid Mind

Starting 6th level, your mind and Ki become less localised from your body, allowing you to spread yourself further, and have greater control over your split self.
Your concentration radius is now 10ft, and any healing received by the prime is also received by any replicas in your concentration radius.

Also, you share your bonus action with your replica, meaning that you can use your bonus action on its turn instead of yours, and if you spend Ki points to use a bonus action such as patient defense within your concentration radius, then the prime and all replicas in the radius also take this bonus action.

Additionally, you can now use a bonus action to have your Ki reserves flow from one of your bodies to another, effectively swapping places with a replica that you can see, or that is within your concentration radius.
Finally, you can use this ability as a reaction to swap minds with a replica when you take damage, so that the health is removed from the replica's HP rather than yours.

Expendable Copy

Starting 11th level, your mind and Ki flow like water, with your body following suit. You are able to expel and sacrifice some of your Ki in order to reduce damage from an incoming attack against you or an ally.

You gain the ability to spend 1 Ki point as a reaction whenever you or someone within melee range of you is hit with an attack that you can see, to remove damage in the same way as deflect missiles; you take any damage not removed, as your Ki took the blow instead.
This ability causes you to split immediately as the attack lands, allowing you to step out of the way as the replica left in your place takes the hit. If the attack was intended for someone else, the replica splits from you and dives in the path of the attack. The replica has no hit points and dies instantly either way. This doesn't require a hit die.

Additionally, your concentration radius increases to 15ft.

Army of Self

Starting 17th level, Your mind and Ki inhabit your body only because you choose for them to do so, and your control over your body allows you to create a 2nd replica, which can be done on a pre-existing replica's turn.
Additionally, your concentration radius increases to 20ft
Finally, If you successfully make a strike against 1 target with each of your selves using Flurry of Blows, you can spend 3 Ki points to attempt to make the target unconscious until the end of your last replica's turn.