How many of legendary pokemons do you keep.

Too damn many. I still need to clear out all my extras seeing as this Halloween event only brought double candy for catches.

Knock it down to 6 at the most. I would even keep less than that for some (Regis, Suicune, Latias, probably Ho-oh). From my perspective, I transferred down to only three or less for most, exceptions being Mewtwo and Moltres. No reason to keep the breakthroughs unless they're exceptional, given the cost to bring them to parity with raids, let alone WB raids.

I would see some occasions that requires a full squad of legendaries to accomplish short-men raids without weather boost or friendship, like Kyogre trio (raikou), Lapras duo (Raikou again), etc. But in other cases you will need lots lots of candies and dust to fully optimize the squad and achieve the task. I would only keep 2-3 high iv ones with good moves and treat others as trade fodders.

Also, in some cases one legendary can serve different roles, like Mewtwo with SB, FB, IB, P, Ray with O or AA, etc.

Only one of each. It makes for a unique lineup and upholds to their “legendary” status. Not very fond of holding duplicates, especially legendaries.

I still have 60+ Rayquaza and Mewtwo. These are legendary. They cost you a raid pass. I wouldn’t transfer one until you make a trade. 800 stardust with a best friend is nothing, and the chance of a lucky or higher IVs. Plus you get 2-4 candies from a trade (1-3 depending on distance and another one if you transfer it). As opposed to just one.

I have never transferred a legendary after a raid, no matter how bad it is. This could be the one you get a lucky perfect from a trade. I would power up a 89% lucky with 15 attack anyway as opposed to a non-lucky non 100% legendary.

Initially one of each.
When they rotate with a special move, I add an extra one. I have no shinies yet, but I would try select the one shiny with special move.

Mewtwo is the only exception. I have 4x SB and will keep one of each for the other moves including HB.

I keep 3 of each for most legendaries, because I like neat rows. Currently holding onto all extra Mewtwos because a friend wants to try a lucky one.

I wouldn't keep any of the last months mewtwo aside from maybe your best 3 or so, they cant get shadow ball, nonSB mewtwo are only useful for machamp stomping

as for others, depends on the individual pokemon

1 or 2.

I only power up one per legendary species, but keep a couple of spares for relevant ones.

I keep between 3 to 9 of each. 3 for those that i dont find useful - regis. 6 for those that are good and worth powering up to the max. 9 because some legendaries had shiny variants and special movesets.

Even with that, i find it excessive, and might do some housekeeping if they ever have double candy for transfer / trade. The only exception are mewtwos... im hanging on to them purely for trading. After trading and transferring, i might keep between 12 to 15 (max).

I keep spare legendaries (that sounds odd) for double candy events. The ones I really keep afterwards are one of each useful species+moveset combination. That means 2 Groudons, 2 Rayquazas, but one Entei for example. Exceptions to this are Kyogre and Raikou, I keep 2 for their usefulness and lack of competition for similar targets (I didn't have any Zapdos until research rewards). Another exception is SA Moltres, I kept many for trades since it's the only raid day with exclusive move I attended. Suicune is the only one for which I even considered transferring my last one, but I kept it for collection.

Once caught 5 Registeels, trashed all except for the 96iv guy. Keep more good legendaries, but trash all bad legendaries except one to keep for the collection.