The numbers game in fitness and Physical Culture for that matter has been a standard for more than a century sense the time of the Mail-Order Courses. Setting a goal to hit a certain number in an exercise or set of exercises has worked for many but others got stopped in their tracks for whatever reason as well. When you hit a certain number, what's next after that? Going for more sets, more reps, greater tempo, what's the endgame here?
One of the gold standards for numbers in exercises (Push-ups, Squats, Powerlifting) has been the number 500. Who in their right mind came up with that specific number? Why is it a gold standard in powerlifting to have a 500 lb Bench or Deadlift or Squat? What about 500 Push-ups or 500 Hindu Squats? Why does that number have significance? It isn't a knock to it or anything, more of a curiosity in something so arbitrary that was made up by someone or a group of people to sell the idea that it is the real number to tell someone how strong or fit they are.
The number 500 is a good number and many people have done far more than that in many exercises especially in Powerlifting. After all, isn't it just a number? Does it have any real historic significance especially since that number has been surpassed more times than I care to count. One of the first guys to Bench 500 was I believe the legendary Bodybuilder Reg Park whom by all accounts was one of the last generation of Bodybuilders to train as naturally as possible before Steroids became the staple of Bodybuilding. When Hindu Squats became popular around '99-2000, 500 Hindu Squats was the gold standard to test a person's conditioning. Matt Furey popularized it but the standard itself was "patented" by Karl Gotch that set the tone for a wrestler's test of stamina and physical/mental conditioning which gave the idea of doing 250 Hindu Push-ups, 500 Hindu Squats and a 3 min Bridge to even get in the door as a wrestler.
Don't get me wrong, I've done 500 Hindu Squats many times from doing a countdown based method, to the 80/20 Protocol (80 Squats, 20 Jumpers) for 5 sets without a break and doing it using a Deck Of Cards that I picked up from an article I read where Catch Wrestler Billy Robinson mentioned the idea of using cards to help with reps. Unless you're a fan of the exercise or want to stay in relative condition for daily life or in sports, it isn't that magnificent of an exercise. It has it's perks and I'll do reps from time to time but after doing 500 so many times, it became boring and it didn't have that spark of excitement to do that many anymore. A few hundred sure, but I don't need some standard to tell me how fit I 'am. If you don't feel a spark or excitement when you exercise, you're just going through the motions and it becomes typical and tedious.
Now on the idea of 500 Step Ups? When it comes down to it, the exercise itself is just like any other that has its perks and drawbacks but for some reason I'm far more drawn to it than the Hindu Squats. Do I think it's better than the squats? Both yes and no. The yes part because I've gotten more out of it than I ever did with the squats and it gives me an opportunity to work one leg at a time which the squats don't. The no part is because in reality, it's just an exercise that I happen to like better, it's not mystical or magical, it's just different. Personally, I'd rather do 500 total reps with Step Ups because it just feels right and gives me a better sense of cardio. I've done 1000 on several occasions (fastest time being 50 minutes) and after hitting that number, it just got boring so whenever I do circuits, I add in those in substitute for squats and would total in most workouts around the 200-300 mark and I'm good. I would also just go for time (10, 20, 30, an hour whatever) and work on sets of 10-20 per leg.
In reality, 500 of anything is really just a number and it's a great one to set a goal for. If you're into numbers like that, that's awesome and if it makes you happy and keeps you strong and fit, keep doing it. What suits you better, 500 Squats or 500 Step-Ups? The real question is, which one better suits your goals and needs? They both work very well and either one will get in you amazing shape so it's not like one or the other are terrible for you, one just suits better than the other according to a person's fitness and possible interests. With the Squats, you can virtually do them anywhere you want cause all you need is the ground. With Step Ups, since it's in the name (no shit sherlock), you'll need something to step up on like a stool, a log, stairs, rock or whatever but you can find those in most places so it's not like you can't find a place to do them. There's also no reason you can't do both, I've done a workout or two using both myself.
Use what works for you and harness the enthusiasm and love for them. Sometimes you may hate them because they're tough to do but yet very simple exercises but in the end, love your exercises even if they're hard because the real benefits is what you get out of them both physically and mentally. Train awesomely everyone and have fun. Training is meant to be an adventure, not a chore or something that will make you feel bored.