As to reducing the amperage, (from the control unit which is feeding the track), this is desirable.
But the real reason you would want to use external power for the m84 is so that they retain their state and for the m83, in on / off mode, to keep the port on. e.g should you have a track short or manually press Stop.

I acknowledge that Marklin do state that a 60822 should be used when powering a group of 60831/2 and 60841/2 with external power (6636*)
Personally I have yet to hear a decent explanation for this and my experience to date shows it is has not been needed.

It is your call, however the the 6636* will plug in directly to the right of the first m8x in a chain - no need for any 60822.

A more realistic issue is whether you can get mains power safely to the vicinity of each group.
Again, your call, but extending the 19V from a single power supply by adding a length of wire and tapping in another plug might be considered.

The overall power requirements for a m84 is quite modest - even with 8 relays on.
A m83 however is a bit more complicated. If you are planning to use the m83 to drive lights or motors that are permanently on then it is conceivable to use up all 2Amps of a single power supply with a single m83 (e.g. 250mA per port x 8 ports)