I don’t think that there is a maximum number of languages a person can learn in general. Of course, if we are talking about efficiency, quality is better than quantity. Even so, everyone is different and unique and the same goes for their learning speed, in addition to how much time they can spend on learning.
As for your question, since you have already started studying Arabic and Spanish, I suggest you focus on improving your knowledge of those two languages in particular. As for the other ones, it depends on how much time you can spend on learning them without negatively affecting the time you actually spend on learning Arabic and Spanish. In other words, do you think that you can create additional time slots for other languages in addition to Arabic and Spanish without affecting the time you already spend on learning those two? Maybe you can give it a try and if you notice that this is what is happening, you can stop the learning of the other languages. All in all, I think that your learning will be effective as long as you can manage well the time you spend on learning each language.
As for your last questions, it is true that learning similar languages can have its pros (for example, if their structure is similar it is easier to memorize it). However, the other side of the coin is that if they have similar words you may start to confuse them. Therefore, maybe at the beginning it is better to avoid learning two similar languages together. However, at the end of the day it depends on you. In fact, if you are very enthusiastic about learning a language and have some time to spend on it, give it a try and see how it goes. In the end, you are learning it because you like it!