I keep three books going at a time. Two are almost always different genera, or fiction vs nonfiction. The last is low priority stuff. The exceptions are where one work really grabs my complete attention, or something that requires intense close study.
My impression of "common people" readers is they fall into one of two patterns. Either uncomplicated novelette sized works, or an uncompromising obsession with a single genera trash series.
>how people manage multiple books at a time?
As harried undergraduates are forced to do, so can you. Unless you are willing to adopt a nineteenth century lifestyle, the greatest issue readers face today is competition for our attention span. Modern trends favoring an attention deficit lifestyle can be combated by bouncing between two books featuring different themes, styles, authors.