Reading several books at the same time - helps to develop imagination. It often happens that one book is very boring, the plot begins to sag, the desire to quit it is already becoming stronger and stronger. Therefore, reading also, another in style, according to the plot. It helps to develop the imagination. Therefore, it is even advised to read several books at once.
As far as I can remember, I have always read 2-3 books at a time. For me, this was only a plus, since I cannot keep my interest on just one piece, the same is with the series. And reading a few books helps to switch. The main thing is that they are not identical in the plot, and then no confusion will arise. And if you have forgotten something, you can skim through the previous pages, and everything will be remembered by itself. But reading just one book was problematic for me - in 99% of cases it bored me, and I stupidly did not finish it.
I read somewhere that it is just the same useful to read several books at the same time. For example: one book on work (will help you get to know the profession better, understand all the nuances and help you climb the career ladder), a second book from fiction, and a third something from science, psychology for general development and raising social skills.
I once read 5, like nothing. Only two of them were artistic. In my opinion, the option is so-so, because it does not make it possible to live by the plot, to be completely drawn into the atmosphere :)
To be honest, it seems to me that during the reading itself you may not have anything confused, but then if you want to remember the plot of one of the books, then it will be able to mix with the plot from another book)
This is, of course, individual and depends on your personal preferences. Some people accept this approach to reading, as I do, and some do not. Here are a couple of arguments in favor of not mixing anything: