How would you use an army of Unseen Servants?

[Dalebert]

You get Spell Mastery at level 18 wizard. You pick Unseen Servant. Every round as you're walking along otherwise bored, you can create an US and assign it a task. How would you put them to best use?

Here's my current plan but I hope you can add to it. While walking through a dungeon, I would create four USs for each PC and position around them, e.g. "Stay just in front of Bob. You, stay just to the left of Bob, etc." At least one of the four would carry a little kit--ball bearings, caltrops, a torch and tinderbox. This would effectively spread them around the battlefield making it unlikely they all would die in a big AOE and also so at least one would be nearby if needed (they can only move 20 ft a round).

I'd have little clusters of them assigned to tedious things, e.g. give a small caltrop bag to one and say "Follow me around and pick up any caltrops within 5 ft of me and put them in that bag." Assign about 9 others to the same task and all you have to do is stand near some and they get rapidly gathered automatically.

With this arrangement, there's almost always an US nearby to give someone a healing potion if they go down. You can assign one near the rogue to always reload his hand crossbow. With any free bonus action in a round, you can send one out to drop caltrops or ball bearings in the path of an enemy, or to spread oil in its space when you're about to Firebolt. You can tell one to light a torch and charge toward a cluster of enemies so you can cast Pyrotechnics. You can have one walk 50 ft ahead of the party to light the way without lighting up the party. You can have one walk ahead of the party dropping a 25 lb weight to test for pit traps and such. It dies? No biggy. Resummon.

[Citan]

I already raised some big problems with the reality of this kind of tactics in the thread dedicated to spell mastery choice, but beyond the general problems you are heavily bending the spell here.

1. No magic items = no potions.
2. Move is 15 feet, not 20 feet.
3. Orders are always about interacting with an object, not something as general as "follow somebody".
-> For caltrops picking example, you would have to use your bonus action each turn to direct one specific US to move and pick up ONE caltrop ("interact with AN object", caltrop is an object -you will take much time cleaning just one bag of caltrops).
-> For crossbow reload example, you would need to repeat the order each turn as well: "reload the crossbow" (since once reloaded, US completed the task so "awaits for the next order", unless DM agrees to such a formulation as "keep the crossbow loaded" to create a 'continuous' interaction with the same single object, a bit devious but acceptable -I'd certainly allow it per Rule of Cool-).
-> For "following someone", it would actually require you to, each turn, tell one US to follow its attributed ally. Making it extremely impractical.

4. Even if a DM was nice enough to allow it (general "follow X" order, total houserule), it would force the character to slow down to allow the US to keep up, or would quickly distance it making it useless.
5. "It dies? No biggy. Resummon". So when it's for US, it's okay to spend a turn casting it? XD Because if outside of encounter, you can probably take the time to ritual cast it. When in an encounter, I thought yourself said the price of an action was hard at that level and better spent on high level spells?

Basically, the practicable and of limited benefit uses you gave...
- Lightning in front of the party, but usually there is someone with darkvision -60 feet- so you are not gaining that much. Worse, you could draw unwanted attention because wandering torchs are usually not a thing in enemy lair.
- Reloading crossbow is classy, but basically useless from what I see: for an "ammunition" weapon, drawing the ammunition is part of the attack so not a problem. And if you have several attacks, it would require the aforementioned lenient DM to agree that an US could reload several times before the next command. For a "loading" weapon, the main limitation is not the fact you need a hand to reload, it's the reloading time. And Crossbow Expert, the feat that solves it, can obviously not be learned by an US.
So in most games, the only situation where an US is good is for a character having only one weapon attack (most casters, Rogue) which he wants to use (barring most casters except Clerics having enhanced weapon attack, left Rogue) with a hand crossbow (barring most Clerics, biaised towards melee) while wielding a shield (so multiclass or feat, or maybe Swashbuckler? Don't remember if it gets additional proficiencies).
- Dropping caltraps or oil is nice, but again, only once per turn, so unless narrow places (for which a lower level spell would probably provide quicker and better effect) the enemies will usually go around (or over) the obstacle.
Are easily achievable with only a very few US, meaning ritual-casting it is far enough. Nothing worth preparing it as a spell mastery.

[Lombra]

Use them to clean up the whole capital of the kingdom in less than a week without much effort :D