How many businesses, income streams, "sources", jobs, etc can you juggle at one time?

NOT how many projects, i.e. if you are a realtor you can have two dozen listings or a copywriter you can have a list of half dozen articles waiting.

my "rich dad" said three, he served on a bank board and committees , rental houses , and farmed 300 acres. said three was all you could do justice to??

How many?