Electrical – How to provide neutral wires for the smart switches

So you have 6 neutrals: supply, neutral to two lamps, and pigtails from the outlet and 2 switches.

If your wire-nuts are not certified for 6 wires, you can use a 6” jumper and nut them 3 and 3 (well, 4 and 4 with the jumper).

Now this will come up if you have two hots coming from the breaker box: Every neutral has a partner "hot" - that's most obvious in Romex 14/2 where you have a white and a black. These are monogamous partners: hots do not share their neutrals with other hots. The reason is circuit protection. If a neutral wire breaks, you want the circuit to go dark, you do not want another neutral to be overloaded trying to handle double load. Neutrals do not have circuit breakers. So if you have two hots, you must be careful to keep the neutrals paired with the hots everywhere they go, so all the current always returns on the partner neutral. This is mandatory with GFCI breakers, since they compare current flow on hot vs neutral to detect leakage.

It is OK to tie grounds together.

Keep 'em separated

Since you have two subpanels feeding this setup (one for each "side"/half), you'll need to make sure that the switches for each side have a neutral return back to the corresponding subpanel, and only the corresponding subpanel. In other words, the two left-side switches have a neutral wire going back to the left-side subpanel, and the two right-side switches have a neutral wire going back to the right-side subpanel.

Cross-coupling the two smart-switch-neutrals in your setup violates 300.3(B)/310.10(H) and creates a neutral current loop that causes inductive heating of metal parts (like your raceways and boxes) as well as magnetic field EMI due to the loop area of the current (old-school picture tubes were susceptible to this in particular).

Neither of these neutrals needs to join with the light fixture neutrals on its side until the subpanel neutral bar, though. The smart-switches are loads of their own (to power the "brains"), and running an extra neutral homerun in the same cable, conduit, or raceway is OK by code, as per 300.3(B) (otherwise, /2/2 NM wouldn't be Code legal, for that matter):

(B) Conductors of the Same Circuit. All conductors of the same circuit and, where used, the grounded conductor and all equipment grounding conductors and bonding con- ductors shall be contained within the same raceway, auxiliary gutter, cable tray, cablebus assembly, trench, cable, or cord, unless otherwise permitted in accordance with 300.3(B)(1) through (B)(4).

Load isn't a concern here, but Code is

The primary concern for multiplexed neutrals is having multiple hots on the same phase return via the same neutral, which causes the currents on that neutral to be the sum of the currents on the various hots. In this situation, though, that isn't a concern as smart switches don't draw much power at all.

However, technically speaking, unless you punch the circuits down as such to have currents cancel (i.e. a MWBC -- such things do exist in the three phase wye world, whether they are two-phase/three-wire or three-phase/four-wire), your proposal violates 200.4(A):

(A) Installation. Neutral conductors shall not be used for more than one branch circuit, for more than one multiwire branch circuit, or for more than one set of ungrounded feeder conductors unless specifically permitted elsewhere in this Code.