The thing is, you're approaching it kind of from the wrong angle. You're asking how you can reduce the number of days per week that you train so that you can just retain what you have and not lose anything. So you want the answer to be a simple "1", "2", or maybe "3" days per week. That's just not going to be easy for you or anyone else to say.
The problem is our brains and bodies don't work that way. When you learn any new skill at first, you're going to have to repeat it 100 times that day. And then you're going to have to come back to it again a day later, but maybe only do it 90 times. And then you can skip a day and come back to it the day after that and only do it 70 times. Then you can skip two days after that and come back to it and do it 50 times. And so on, until you can come back to it a month from now and only have to do it a half dozen times to retain that skill.
That's how our brains actually work. And this is what's known as the "Spaced Repetition" method, which you can learn about here:
The Spaced Repetition method can serve as the framework for figuring out the minimum time needed between training sessions to retain whatever you want to retain. It's often used to learn new vocabulary and new languages, but it can be applied to any skill in general.
It's not as simple as limiting your workouts to one or two days a week. Some of your skills will require more days training per week with less number of days resting in between. Some skills will only require training once a month. And the number of times your repeat each skill will depend on the previous times you've done it and whether or not your previous attempt was better or worse than the ones before it.
I talked about this much more in depth several paragraphs down in my answer here:
What is the most effective way to teach Jiu Jitsu
What I would do if I were you is to prioritize what skills I want to retain. Once those are prioritized, you can focus on them and determine the frequency (how many days on, how many days off) and duration (how many reps) of training.
Start with a rough guess for everything you prioritize. Write it down. Program it. Then after two workouts, you can determine if things got better, worse, or stayed the same. If things got better, you can afford to space the training out more and/or reduce the number of repetitions or duration you train. Then go back to your program and make the changes. Always update it after every workout.
Think of it like a big feedback loop. Your performance today will change the frequency and duration you program in for the next time you practice that skill. So if you go to practice your uchi-mata throw, and it has gotten worse than the last time you did it, that means you need to increase the frequency of how often you train it as well as how many repetitions you do each time you train it.
It sounds like a lot of work, and it doesn't give you a simple answer of 1, 2, or 3 days a week like what you really want. But actually, it gives you the right answer, which is the minimum effort you need to put into any skill in order to retain it.
If that answer doesn't work within your own personal time limitations, that's where you must change your prioritization. Prioritize that skill more highly, allowing you to focus on it more and spend more time on it. But that will mean that other skills will worsen, unfortunately. That's okay. It's all about your own priorities in life. Nobody has infinite time and energy to train.
Hope that helps.