So Disguise Self but on others.

...and lasts longer, and can be cast on multiple targets.  ...but yeah.  "Disguise self', but better.  Use it to make your entire party look like foot-soldiers of the BBEG, or magically paint them all in hunters camo while traipsing through a forest.

It's 'any number' of creatures within range (30ft), so if you all squeezed in tight for just a moment, you could probably disguise a whole village of people!  Imagine the enemy horde of zombies just casually walking by the dozens, perhaps hundreds, of skeletons that occupy this now seemingly dead ghost town.

The spell needs additional tags, please. "Deception" and "Shapechanging" would make it easier to find. (I couldn't remember the name of the spell and spent half an hour searching for it.)

Yes, this is a great spell.

"Use it to make your entire party look like foot-soldiers of the BBEG..."

Or like cows to lure a roc. 

Use it to make an advancing army in the midst of a chaotic battle, look like their enemies so they start killing off each other.

Okay, so how *I* would rule it, the Seeming would remain in place regardless of separation from the caster and distance, but based on prior experience with reading spells, it should be asked: Is this an area of affect as in an aura sort, where if people was outside the aura the Seeming disappears?

Every other aura type spell that I'm aware of specifies that it emits something from you to a certain radius for the duration, and then any creature you choose within that radius gains the benefit. Seeming makes no such mention of something similar, nor specifies anywhere the effect ends if a target was to leave the range, so it most certainly remains in effect regardless of the distance a target travels from the caster.

very peculiar cows 

These wild, humanoid cows

Quote from dmjonesie >>

The spell needs additional tags, please. "Deception" and "Shapechanging" would make it easier to find. (I couldn't remember the name of the spell and spent half an hour searching for it.)

Yes, this is a great spell.

 it's not shapechanging

This is a great "get away" spell. Running from guards? Run to a crowd of people and then make a bunch of them look exactly like you. Walk away, or follow people as they leave to figure out what's going on.

Quote from JoeyFromSchool >>

These wild, humanoid cows

 Minotaurs then.

Any number of creatures, not specifically just persons...
"About 80% of that pack of Crag Cats now look like Dire Wolves; let's see how their pack-mates react!"

(& its an area effect, not targeting any specific Crag Cat alone, so they wouldn't get to try Spell Turning)

Reminder that this works on unwilling targets, can use it to turn the parties appearance into enemies in the middle of a fight for maximum confusion. 

Notice that this spell does not include the same line from Silent Image "If a creature discerns the illusion for what it is, the creature can see through the image."

You also get one initial save for avoiding the illusion effect. Successful Intelligence checks only let you discern that the target is disguised.

So you can target any number of hostile creatures in range and make them appear to be wearing a solid metal helm, obscuring their vision and effectively blinding them. The targets get one charisma save and if they fail they are blinded for 8 hours with no concentration required.

The best use of this spell is to turn every minion in the BBEG fight into the BBEG and just let the players panic. 15 Strahds running around Castle Ravenloft? Yes please.

You can't change a target's body type, so you must choose a form that has the same basic arrangement of limbs. Unless you already a quadruped no 

Cow: has 4 limbs 

Humanoid: has 4 limbs 

I rest my case.

Considering how odd it would be to have cows walking around on their hind legs, if the players want to crawl around prone on all fours that badly, why not?

It does say similar arrangement of limbs. Bipedal vs quadrupedal. I would not allow that.