Are these tight when just worn round the neck?

I found them too loose (ladies average neck), so I didnt like them. Depends on the fit you want.

is this item single ply

Yes they are. I purchased to wear to keep my hair held back not tied in a bun. Keeps my hair covered, breathable and easy to slip on. Hope this helps.

Are these actually seamless. Reading some comments in the reviews that they have seams

It means seamless at the ends - no hem.

I like the navy multicolored muffler? How many mufflers will i get- all 12 ir 1? If 1 muffler how do i know which one?

You will get ALL 12 .... I use them daily, either as a band to simply keep my locs back or to cover my locs to prevent it from attracting dust after being washed, and because it is breathable my locs remain fresh until I get back indoors; in the event I needed to go outside briefly. They are colourful so matches most of what I wear and seems to have kept their elasticity so far although some of the patterns, like the ying and yang symbol and the sun with a face on it, are very much not to my liking and not what I would wear on my head, for the price it was worth to even only have about 4 or 5 out of the 12 that I kept and could use. Anyway, hope this helps. Ingrid.