LOL...custom ADD.
Well, up till recently I've only actively worked on one at a time. That doesn't mean I don't have something else started...but that it's just put away till I finish what I'm focusing on. but I need to work my way through a waiting list, and so I've actually got 3 active customs on my desk right now. Now if I go and count the customs I've started and got set aside that I'm not working on, I think I have 2 more.
As far as planning...lol...story of my life! I plan everything to death! Inspiration strikes me at the oddest times, so I've got this notebook I keep all my ideas in. Each figure I want to do has a page where i list all the physical things and colors I want to use, and also every piece of gear I need and what needs done to that, and I check things off as I find them. And when I find a cool piece of gear I go through each figure first and see if it works for any of them so I can earmark it for a custom or for fodder. Sometimes gear will give me a figure idea, but mostly I get ideas from characters I really like and start planning away.
I think I have 5 planned figures on paper right now. The only figure I don't have fully planned is an original character, because I'm rewriting his backstory. When I first started customizing I would not start the custom until I actually had every single thing I needed. Not so much now, tho I do like to have most of what I need.