It's low in terms of numbers, and I'm also assuming you're not counting HZ, so yes, four, but there's a lot to consider. Whistle, Rookies, Rising Impact, Hikaru no Go, and Prince of Tennis all started between 98-00 and all had most of their run in the 2000s, so there was a massive influx going into that period. You've got to consider that having all these running would have made it harder for new series to take off. 2003 for example had 6 hit sport manga running at once (though 3 ended in the same year). Note that this was during one of the most competitive times in Jump history. You could also argue that Yu Gi Oh falls into this category, being quasi board game/battle manga that satisfies a similar itch, and which started and ran in a similar period. Eyeshield and PoT then remained dominant for most of the 00s, running 7 and 9 years respectively, cutting out other sport series until they were reaching their conclusions - around the time Kuroko started.
There is certainly a problem after this though, since Kuroko ended in 2014, and HQ is the only one to take off since. That said, HZ has probably played a significant role in this. In it's 5 year run, how many potential manga has it caused to be cancelled by occupying a place? You could argue they'd have let another sport manga survive if it wasn't around, like say RxL, which even at its worse was still selling roughly 2x more. Maybe nothing else would have taken off, but you can't deny the possibility. Now that it's conclusion is mere weeks/months away, it will be interesting to see what impact this has on new sport manga.
Also interesting that many of the not-insta cancelled series post 2010 have been sports manga. Kurogane at 68, Cross Manage at 42, Straighten Up at 86, Mononofu at 43, Robo at 62. These make up 5/9 manga post 2010 that made it past 40 chapters but were cancelled without taking off. They're certainly trying at least.
And then sales. Jump sales rankings in the 2010s (ignoring OP & HxH from 2 decades prior) are AC>HQ>MHA>Kuroko>>>rest as far as peaks are concerned. Not to mention that a sports manga (or multiple) has almost always been near the top of the Jump lineup popularity-wise throughout pretty much all of the magazine's history. The genre is still very much important and valuable to Shueisha, and I don't think they’re even close to giving up on them since they're releasing 2 of them just in this one round. New sports and comedies are easily at the top of my “want” list right now.
