That is a good question that I too once wondered. Based on my experience as a moderator, under normal circumstances, you can't lose your rank.
I can give an example of why that is so. I have the rank of "Ace". To be an Ace, 3,000 points and 15 best answers are required within the past thirty days, once I gain that rank, I cannot lose that. So this past month, after getting the Ace rank, I got less than 15 best answers, but I did not lose my rank. Likewise, with points, if you get less points than the minimum of your rank, you do not lose your points. Once you gain that rank, its permanent.
There are a few exceptions in which you may possibly lose your rank. But it is not possible if you are a normal, rule following user. The only time I have seen people lose their rank is if they get their account deleted. But do not worry. People only get their account deleted at the most extreme level (such as using profane language and spamming dozens of questions/answers) In which case, they do lose their rank, by deletion.
So based on what I said, do not be afraid. If you follow the rules, even asking dozens of questions that make you fall below the minimum amount to your rank, as long as you have at one point gotten that rank, you cannot lose that rank. I hoped that I answered your question to your preference.
Another note I want to make is that If you have any future questions involving Brainly whatsoever, do not hesitate to ask any of us Moderators. We are here to help.t rank