Will prices still sometimes go down and a slight increase to preswade.

Is there a cap? My charisma is around 13.

Prices have a cap, you do get more settlers and companions get more health.

What charisma level is the cap?

Oh it's 16+

their is no cap on Charisma but prices have a cap at 80% total value on selling, 120% on buying.

Noticed that, don't have to change clothes anymore to higher charisma stuff. Prices still the same.

Prices cap out at 0.8x base value when selling and 1.2x base value when selling. These caps are achieved at 16 Charisma, but collectibles and perks drive this down. With 2 ranks of Cap Collector, the Barter Bobblehead, and all 8 issues of the Tales of a Junktown Jerky Vendor, the cap is reached at 10 Charisma. With 11 Charisma needed to guarantee success in Speech Checks, your end-game Charisma doesn't need to be any higher than 11 for anything your character does directly. Higher Charisma values will let you have more settlers at each settlement, so higher values are worth getting if you're investing heavily into the settlement system, but if you're not, there's no real reason to go past 11 once you get all the barter-related collectibles and both ranks of the Cap Collector perk.

In addition to prices (which won't matter by end-game), the higher your Charisma, the more settlers you can have. (A settlement is capped at Charisma+10).

Also, if you want to guarantee success on a Red-grade speech check every time, your Charisma needs to be 11. (10 will still fail 14% of the time.)

If there IS a cap, no one seems to have hit it yet. I thought it might be 20, but someone tested that for me and managed to hit ~23, so that evidentially is not the case. Who knows how high you can truly go? You'll get diminishing returns on caps I imagine, and persuasion stop improving at 11, but you can flood settlements with nameless clones.

Is 11 guarantee for speech checks even if option is red?

This is interesting, an nice hat and clothes, fashionable glasses and a beer gives an +5, You need 6 charisma for local leader anyway so this is pretty trivial.

21 is an decent amount of people at an settlement anyway. Nice with lots of people at main base but hardly see the reason for +20 anywhere else except role play.

say 2-3 farmers, one for the trade network one for scavenging and one for artillery.