Your custom opponent can have as many clothes as you want. There's no limit in the game code to stop you. Just list all the clothes in the properties file and start making those ridiculously long "0000011111" folders to put your images in.
That being said, it makes for difficult, long and frustrating games when the ladies are dropping their earrings and socks one at a time. I find it a lot more enjoyable if there's tangible progress at each step. Check out Stripe's coffee nook opponents for good examples of players that have 7 clothes but are still very fun to undress.
Note that making the opponent non-linear isn't that huge of a burden as it might seem, even on an opponent with a huge pile of clothes. You can limit the flexibility to occasional binary choices along the way. And you don't even need to render all the steps if you're willing to re-use poses and do a little photo-shopping. That's actually handy even in the linear case: just render one image of the player fully dressed, one in their underwear, and one completely naked, and then paste bits around until you have 7-8 images for the time cost of only 1 pose setup and 3 renders.
My final recommendation though? If it's your first custom opponent, keep it simple. 4 clothes, fully linear, simple environment. You're going to be doing a lot of learning. Save the fancy stuff for your next one.