Ok I set up 3 bramin troughs at sanctuary but only 1 Bramin showed up. How can you attract Bramin to your settlement if there are none there?

I only have one at each settlement so far, I don't know of a way to get more than one, I don't know what particular action I did made them arrive as I didn't put the feeders down until after they arrived, mostly to stop them standing in the doorway!

I know there is a random wandering trader that sells them, can't tell you where to find her as she wanders everywhere, I also cannot say whether or not you will have more than one at the settlement you send it too. I don't know if there is a limit to how many you can buy, maybe just one.

Sorry I can't be of more help.

There is someone out there who will sell you a brahmin.

At Finches Farm I ported on and the damn things were all over. After setting up troughs saw had 4 of them.

It seems more or less random, but there seems to be a few factors that can increase the odds of getting one just a bit:

Having a fenced in enclosure: I don't know why, but all my settlements where the Brahmin are fenced in get more of them more often.

Having one Trough per Brahmin: Again, settlements with more Troughs seem to get more Brahmin (eventually), up to the number of Troughs.

Having a high Population

Having a large settlement area

Settlements that have the "Farm" icon on your Map

Higher Happiness

None of these are guarantees, but after a total of nearly 500 hours over multiple characters, this does seem to generally tip the scales toward more Brahmin in a settlement.

Some how at Hangmans alley I ended up with two. I had to put their feeders outside the walls because I couldn't move around. At one time I had 16 settlers there also. It was very cramped.

yup. Hangman's alley is weird for me too. as soon as I took it, had 3 next day. no trough.

there must be rules to it as Jorifice has stated.

just don't know what they are exactly.

It's weird they always turn up for me once I get a few settlers, I just thought that was normal, seems a few people are having trouble with them. If it's any consolation they can be a pain!

Well I am no Brahmin baron (YET).

BUt I think some settler's when they arrive indeed bring a mu-cow with them and it sort of is tied with them if you can figureout who has it you might be able to relocate it. I am still testing to see if indeed wild ones can be caught (certain jerks on an overpass take pot shots at them all the time which might explain them running all over finch farm)

I have bought one but it seems a tad hit or miss if this works and you actually get them

That's been my observation as well: they arrive with a specific settler. Theoretically this means if you leave your beacon on and get new settlers all the time (by sending some away or killing them) you could have lots of brahims. Haven't tested it because brahims annoy me. The fertilizer is nice for chemists or demolition experts, but in small settlements they're annoying.