I have between one and six...
...at every settlement I've built...
You don't need any. OTOH, it's good to have at least one suit for emergencies.
If you're someone who relies heavily on it a couple different suits tuned in different ways is useful.
I haven't found more than 2 or 3 suits tuned up and ready to go at once to be useful. Any more than that is overkill, as you'll simply not need that many suits of power armor. I don't suggest putting your companions in PA as they tend to get it broken very frequently as they do not use cover anywhere near as well as you will.
Possible Options:
1 suit based around strength and melee.
1 suit based around using a jetpack and AP regeneration
1 suit based around using VATS and AP regeneration.
I have about 18 sets. I lined them up on the roof of the Red Rocket. Four are empty frames, Two Raider, two T-45s, two T-51s,Two X-01 the rest T-60s
I only use two of them. One is maxed out X-01 lead lined with targeting HUD and the other is Danse's set with a jet pack. I have 75 Fusion cores.
This playthrough I am collecting them but in the past I stopped at 5 or 6 since you don't really need more than one even if you use PA a lot.
How many do you "need"? Zero. The entire game can be played without PA.
If you do think you need PA you still only need one suit, preferably of X-01. Assuming you have the crafting perks you can then tailor it for any mission you want, over and over again.
But for those of us who use PA as status symbols and decorations though the answer is "every suit I can lay my hands on". I currently have 15 full suits, most of them T-60s but at least one of each type, done up in all the different paint jobs and displayed around a large room I built as an homage to the movie version of Tony Stark.
Even if you use PA all the time you only need one as you can easy swap armor pieces, however you will find frames around so why not collect them
I have 16. Which includes two sets of X-01 armor.
Since finding the X-01 armor that's all I use.
"Need?" 0. At least 2 X-01s, one for you and one for your companion, and 2 T-51s that you can use when you don't want to spend an arm and a leg in repairs are good to have, but I haven't felt the need to use them much yet.
I stopped after having two suits of X0-1, and one each of the others. I have one suit with a jetpack, and the other with Tesla Coils. My BOS T60 is just for RP, with the BOS chestpiece, and a T51 with the Cambridge Labs chest piece. I no longer collect frames, but I do like leaving empty frames with fusion cores in them for settlers to jump in when under attack. (that always cracks me up) Now I just strip frames in the field, and sell the parts.
I only use one suit.
All the others get upgrades, paintjobs, and snazzy warehouses to display my collection.
I like to have one of each type on display, raider, T-45, T-51, etc. After I get my Armor skill maxed out I'll upgrade them all to the max more for the EXP than anything else. I never use the stuff.
I don't need any.. I don't like it.. It's uglier than before and now it has the annoying need for some ammo.. really expensive ammo.. No thanks.
That is like asking how much money is too much or how much six is too much?
I've collected a huge load of suits all over the place, but I've only ever used one set - a lead-lined T-60 when I need to go to the Glowing Sea. The rest are all standing as silent sentinels in several settlements just to look cool.
I have maybe 30 sets? I know for sure it's more then 20 sets intact ones, I just got my forth X-01 set. I like upgrading them all for different purposes and paint schemes.
I have a few suits of armor, basically one at each of my major settlements. I like keeping fast travel to a minimum, so having a spare suit comes in handy.
What kind of stupid question is this? You can never have enough Power Armor.
besides the ones i have in my settlement for bragging, i also have one on the bottom of the sea. one misstep, and you go down fast. couldn't be bothered with walking back to the shore underwater, so i just exited and left it there for future generations to find it.
Seven.
Change PA when you change underwear.
(or twelve, if you are a recycler)