Just one, but it's fairly new.

My Simming time is quite limited (about one day per week of actual game time). So, I'd never feel as if I'd accomplished anything if I had more than one neighborhood. Especially, considering there are currently eleven neighborhood households, two university households and I play a seasonal rotation with each (uni is played on a semester rotation).

I have one - Carnen Bridge. The neighborhood I downloaded from here! So, technically, I didn't make it, but it's what I use for my custom hood! (so, does that count?)

I have had issues with constantly restarting neighborhoods. After downloading Carnen about 2.5-3 years ago, I played a CC-free game and fell in love with all of the families and the stories I created. Unfortunately, the hard-drive on that computer conked out. I was able to get most information off of it, but for some reason everything was jumbled up when I transferred it to my new computer (from what I remember, it was mainly names, but there were signs of hood corruption before the hard drive failed. I had some weird glitches but had played around them). I couldn't really play it anymore, though.

I decided to start over again, but didn't feel like playing them all again. I have made an extensive history now that goes back 3-4 generations before those old families. I've re-started so many times, and I want to see how far I get. But if one thing went wrong, I would re-start all over again. Now, I'm trying my best not to re-start. I don't play too often so the neighborhood is growing very slowly. I only have seven playable families at the moment - I can't imagine how slow everything will go once Generation 2 is here (plus new families coming in to the neighborhood). Man, when will I get to Generation 5? That is the generation that I first originally started about 3 years ago. So, yeah, hopefully I don't feel the need to re-start...

At the moment now my custom 'hoods are:

Skaia - my Homestuck neighbourhood (New and uncorrupted as yet)
Herning - my good old Legacy 'hood. :') I have at least 20 backup Hernings!
gkuhduki - an unnamed new hood where I will resurrect all the sims that aren't a VBT to resurrect (such as the deceased OFB sims from Bluewater Village and the Tricou family, according to Sims Wikia)
Godric's Hollow - my Harry Potter themed hood. <3

I think I've got another but I'm not sure what it is called or what is in it. Probably deleted it.

I had one custom neighbourhood, but then my hard drive failed. Backups FTW. It's just the main hoods for me now.

MOOKIE, your neighbourhood is stunning!

@simsfreq , thank you, much appreciated!

I just have one as well.

In the past (I've been playing this neighborhood for 2+ years), I had multiple hoods, but found that things tended to be sporadic and that choice, ironically, would led to boredom and too many half-started families, projects, etc. I've kept this one going strong by focusing solely on it.

I finally have a proper playing hood since last week I always had this dream of building my own custom hood, but I seriously get distracted by all the other creative projects I want to do (recoloring, meshing etc). So for the first time ever, I went on a lot downloading spree, plopped them all on an emptied terrain, tried to put them in a location that sort of makes sense and now I get to do make-overs for a lot when I finish one week of playing Then I also have a building and tesing hood, but that one doesn't really count as a proper hood.

I've made quite a few. Simstown was my first but it's gone since I lost it on my computer, it was going to blow up eventually anyway. Now I have Springwater Station, which I love. I love some of the buildings I've made but mainly it's the Sims. I'm currently working on a new Springwater Station though where everything will be made from scratch (except I'm not sure about vacation hoods yet, might just use the normal ones) including the terrains. I already have my starting families' names picked out. Piccolo will be the name of the founding family of course, I can't have a hood without my Piccolo name. Ferry will be the name of a middle-class family, because I love my current Ferry family and like the idea of using Disney/fairy tale first names. Latina will be the name of a low-class family and all of their names will be in Latin. I want them to have a lot of kids and not a lot of money as a challenge. Rags will be the name of my other low-class family but they will be easier because I want them to live in an apartment with one or two kids and try to build their ways up with a business. The husband or wife can sell the objects he/she sews.
So far the first names I plan to use are Daniel & Kathryn Piccolo, but I may want to change Daniel as I used it last time in my other Piccolo family, then there's the classic Phillip and Aurora Ferry based off of Sleeping Beauty, then Cornelius (a common Latin name) and Margarita (as in Margaret or the word for "pearl") Latina, and George and Penny Rags- if they have a son I want his name to be Dustin Rags. I'll have to come up with other names that fit the last name though when the time comes.

I used to have 1-4 custom neighborhoods at different times, and also played a bit in Pleasantview and much longer in Strangetown.

I only ever have one at a time because I barely have time to play as it is. My all time favourite was Crystal Springs which I played for eleven generations. Those Sims were my babies, but then my game got corrupted somehow and some of my Sims had white (as in the actual colour white) skin with text on it.

Currently I'm playing Guillemarre Island, which has only recently been created and has some fun families - the founder and her descendants live on a pirate ship.