While it is hard to do and particularly hard to do while starting out, the general conventional wisdom (and wise it is) is that one should try to have three pieces under review at most/all times. Why? Because academic review is a capricious enterprise that often takes much time. Journals have gotten much better about shortening review times, with many journals averaging something like 40-60 days... which is actually more like two to three months (do weekends count?). But that is an eternity for junior faculty, and the reality is that rejection means that one goes through this spin cycle...