Best formation/size for "City Guard?"

[Beastlord Rakarth]

While I am aware there are better core options available, I really love the look of a legion of uniformed spearmen and crossbowmen. My question is what are good unit sizes and formations? I'm sure this has been discussed before, but my search results were not successful.

As an Empire player, I usually field 50-man units in horde formation which have performed well against my regular Vampire Counts opponent. The extra attacks help to get a higher CR bonus so that more of them are destroyed from unstable. Would a 50-man Dreadspear horde be viable?

I assume that Darkshards would be best in 10-man units with a musician, but am curious to see how others field them. I could see having a horde of 20 and reforming them when about to receive a charge to get rank bonus.

[Blackfrost]

Hello there
I find that thirty five to forty spears is a good size for my meta while dark shards function best in either groups of ten or twenty.
I use spears for an anvil of sorts going five wide. The shards I go seven wide for volley fire as going horde with them would take up the space I need to fit in my other units on the field.

[Calisson]

The only thing to add is that shards should get shields & FC in large units, whereas they don't need it in small units, where the muso is enough (for the ability to rearrange and shoot).

[Daeron]

I assume that Darkshards would be best in 10-man units with a musician, but am curious to see how others field them. I could see having a horde of 20 and reforming them when about to receive a charge to get rank bonus.

I have seen units of 10 used very effectively against me. Especially if you have a sorceress, who can then hop from one unit to another. Still, if you plan to field a few units, it wouldn't hurt to have one slightly bigger rxb unit in the middle where the sorceress starts.
Optimal sizes in my experience are 10, 14, 18, 22.

A key factor to help you decide on size could be the mobility you have... Or not have. If you lack swift mobile units, since city guards tend to focus on infantry, then small rxb units can help you reposition more easily... Or they can play a speed bump. I would give them shields in that case, to make them a bigger threat.

Will you be playing with an eternity king list, or regular dark elf list? The EK list would permit your RXB to shoot in three ranks instead of two, and the spears get an extra fighting rank as well. That would permit the dreadspears to deliver 50 attacks with 50 models in horde formation. It's a power upgrade for sure, but ET rules can also make your opponents stronger.

If you're not playing ET rules, I would cap the size of the dreadspears at 40. If you want to make it a combat effective unit, a combat hero or lord on foot can really boost that unit. I haven't got much experience with it myself, but it was recommended to me by a casual tournament player.