There isn't a completely "right" answer here.
Last year, there were a total of 1131 positions in SOAP. In the first round, 580 positions were filled, and in the second round 284 positions were filled, for a total of 864 positions. On day two, all rounds combined, 161 positions filled. On day three, all rounds combined, 8 positions filled. At the end, 98 positions remained unfilled.
So, we can see that:
Day #1: 76% filled
Day #2: 14% filled
Day #3: <1% filled
End: 9% unfilled (unclear if these positions ever filled, or if they filled outside the match)
Given this, you have two options:
1. You could apply to 10 programs at 12 noon. Presumably, all programs with open spots will continue to review applications. Some programs will have their offers rejected that first round. So, although you'd waste some applications, you'd also (perhaps) be reviewed by programs while they await answers from their first round of offers.
2. You could wait until 2PM, when the list is updated. You would not waste any applications. But, since round 2 starts at 3PM, it's unlikely that a program would be able to download your application, review it, talk to you, and rank you before 3PM. Perhaps that could happen, but seems unlikely to me. So although you'd not waste any applications on programs that have filled, you'd probably lose any chance of getting reviewed for Round 2.
So, in the end it's not clear to me what the best strategy is. If programs review new applications while awaiting their round #1 results, it might be better to apply early as there are still quite a few spots in Round #2. If most second wave applications are reviewed Wednesday after 5PM for round #3, then it's better to wait until you have the updated list after round #2 to apply (or at least after Round #1). But by then most spots will be gone.
Part of me thinks that neither is really going to matter, and that the first 20 applications are all that really matter. That's not based on any data, just my gut instinct.