Pretty sure I did about 200,000 steps in one 24 hour period, just over 3 years ago.
It was during a 100 mile race that took me just over 23.5 hours. My running watch measures cadence, not steps. For the first 8 hours, I averaged 165 steps/minute. I borrowed another watch that doesn't show cadence for the next 9 hours, and got mine back for the final 6.5 hours. I averaged 136 in the last part, because I was walking a lot more. The middle section pace was in between, so 150 is a reasonable guess for it, and the entire race. I'm not sure whether the watch factors in non-moving time or not. I had it in race mode so it shows the actual time, but I'm not sure about cadence. Let's assume it doesn't. I figure I was moving between 22 and 23 hours. I only sat to change socks and shoes, and a couple potty stops, so even at the aid station stops I was still walking around a little bit, and a little bit of walking before or after the race with the watch off, to fill out the 24 hours.
150 steps/min * 60 min/hr * 22 hours = 198,000 steps
150 steps/min * 60 min/hr * 23 hours = 207,000 steps.
That 165 cadence is pretty typical for runners. I have pretty short legs so I tend to be on the high side for my pace. Just saying, what the watch is telling me is probably accurate.
I remember estimating before my other 100 miler that I would be landing on each foot about 100,000 times. That one took me about 26.5 hours, more than a day.