Well, I'm going to piss on your cereal a little bit here and tell you that there's no hard and fast number for any sport. Truthfully, it all comes down to movement quality and what you can do with that strength.
Let's quickly define what I mean by "strength" in this context: I'm talking about your maximum numbers in any exercise that can be traditionally compared from athlete to athlete. Think squats, deadlifts, cleans, snatches, barbell and dumbbell pressing, lunges, rows and various upper-body bodyweight exercises. If you find yourself in a situation where you're talking to another coach and comparing athletes bosu ball squat numbers then excuse yourself immediately, pay a homeless guy five bucks to punch you in the neck and then pay him another five to go punch the coach with which you were previously speaking in the neck as well. If you're unsure that an exercise can be easily compared to other athletes then you shouldn't be worrying about that exercise quite so much. Don't major in the minors.
So, back to "how strong do you need to be?". Well, what's your sport and what's your position? This is an answer that can vary quite a bit. Does a basketball player need to be able to squat 500 pounds? What about a football offensive lineman? What's a good bench press number for a baseball player? What kind of numbers should you be trying to achieve with your athletes? Dan John has written several times about his Strength Standards for various populations, but do these always apply?
One of the first things that I should mention is that as long as you're working with athletes who are active in competition (i.e. someone in high school, college or post-grad athletics) you should essentially have no concerns about maxing out someones genetic potential for strength. If you're managing your athletes well then there actually won't be a ton of time to focus solely on increasing their maximum strength. When you're dealing with pre-season, in-season, post-season and early-pre-season meso-cycles you're going to need to spend time chasing a lot of different training adaptations. You'll need your athletes to recuperate, gain weight, get stronger and get faster all at different times of the year. If you find yourself chasing all-out max strength numbers for too much time each year you're going to be doing your athletes a disservice.
So, again, I ask the question "how strong do you have to be?", and the answer continues to be "strong enough to do your job". There are some outliers out there in team sport land who are freaks and can just move a ton of weight; don't worry about them. If you're a football lineman and your best squat is 300 pounds (and you weigh 300 pounds) then you should consider getting your strength levels up. If you squat 800 pounds (holy shitballs) then your strength levels are well above what you can realistically apply in a sport specific context and you should concern yourself with other aspects of training. With that being said "too strong" is something you rarely have to be concerned with.
That's 5'7" 165-pound pitcher Tim Collins doing a set of forward lunges with 300 pounds; not exactly baby weight. Is this too strong? While I'm sure that Tim is capable of squatting a ton more, his sport is a single leg sport and increasing his strength in a single leg pattern will certainly help increase his throwing velocity.
In keeping with the theme of strong-as-fuck baseball players, here's a video from my buddy Dave Rak at the University of Washington of his 3rd baseman hitting a fairly routine set of 3 RDL's at 160kg/352lbs.
The impressive thing about both of these lifts are that they are essentially accessory movements (true that they can double as main movements). An athlete that can RDL or lunge that much weight with a good movement pattern and crisp tempo (as exhibited) is certainly going to be able to transfer that weight into a sport specific context. Will it be beneficial for the coaches to spend the time working these athletes up to a 450-pound lunge or RDL? Probably not, and I know that neither of the coaches responsible for these athletes are writing programs designed with that point of increasing absolute maximum strength in just a few lifts in mind. They are, however, writing programs that will gradually and consistently increase those numbers over a period of time. There is no hard-and-fast strength number for athletes; you don't hit a particular benchmark and then just stop training because you're suddenly "strong enough". It's up to the coach to determine that you've achieved an appropriate quality of strength in one (or several) movements and that your time would best be used by focusing on increasing other qualities while maintaining that level of strength.
Finally, there is really no such thing as "too strong", as Mark Bell says "strength is never a weakness". Nobody has ever said "damn, that athlete was just too strong to make that play". Focus the appropriate amount of time on making your athletes strong as balls and everything else will fall easily into place.
Have a great day and go lift some heavy shit!