To master Data Sufficiency, find a resource, whether a course, video, or blog post, that covers some DS basics, such as how to simplify the question, how to analyze the statements, and how some typical traps work.
Then, practice a lot until answering DS questions comes naturally. I personally had trouble with DS at first, but after I did a few hundred questions, I was comfortable with the question type, and I'm sure you'll have a similar experience. Just be sure to practice untimed at first to get yourself time to learn to see all the angles of a question before you seek to answer DS questions at test pace.
Regarding MSR, it's a lot like RC and CR except with more information overload. So, your first step for MSR is to get super strong in RC and CR. Then, use some resources to get some insights into how to handle the information in MSR questions, such as the MSR chapter in
TTP, which you can access by signing up for the trial. Finally, once again, practice a lot.
Regarding how many questions to do, quality will beat quantity. That said, you can probably do 20 or more DS questions in a day, not that you necessarily have to do that many. MSR is a different story. I'm not sure how much time you have per day to prepare, but in any case, I think as few as 3 to 5 MSR prompts with their associated questions could be a decent amount of MSR practice in a day. You can see how you feel though, and once again, untimed practice until you're achieving high accuracy followed by working up to test pace is the path to mastery.