I didn't realize they ran out.
They don't cut off sales of
Genie+ — they'll always take your $15 — but the number of reservations for each ride is limited throughout the day. So reservations for popular rides (Slinky Dog has become infamous for this) may get snatched up much earlier in the day than less in-demand rides.
Sort of assumed you could schedule one ride per every two hours.
You can, but the rides you want may not always be the ones available. So far, I haven't heard of
all rides in an entire park running out; it's usually only a few of the most popular ones. Of course, that might change as the usage of Genie+ increases.
Still not sure I understand what "stacking" is?
It's when you're able to hold reservations for more than one attraction at a time, usually from combining both of the main "triggers" that make you eligible to select your next one. You mentioned the every-two-hours rule above, but you can also get your next reservation after you've checked in at your Lightning Lane for a reservation.
The two-hour rule applies to the time at which you make your reservation, not the time at which you're supposed to return to the ride. So if you reserve something popular in the morning, and G+ gives you a return time many hours later, you'll be able to make your next reservation two hours from that point, perhaps for something with a much sooner return time. Then, once you've tapped in for that, you can make another reservation, and so forth, all while still holding onto that first late-in-the-day reservation you made, and possibly others. If you combine short and long turnaround times strategically, you can end up with a lot of popular rides late in the day.
For an additional twist, you can also make a new reservation if you let a reservation window close without tapping into the attraction. However, you can still enter the attraction for an additional 15 minutes (basically a "grace period"). Some people use that to stack even more reservations, reporting that they made one reservation after letting the window close
and another after tapping into the ride during its grace period. This might be an unintended loophole, though, and we shouldn't be surprised if Disney closes it.
Regardless for our upcoming trip we are leaning toward not using it. We aren't ride commandos. We have our favorites, but like just being in the parks and chilling out. Probably why we like Epcot so much.
Most people say that it's not that useful at Epcot unless you're park-hopping anyway, since it doesn't have a lot of attractions, and most of them don't get super-long lines. Same goes for Animal Kingdom. As for me, I'll be using it for my upcoming trip, but that's because I haven't been in 20 years and I have a lot of catching up to do. If I went to the parks regularly, I probably wouldn't. It's definitely designed for some people more than for others, and it actually wouldn't benefit anyone if that weren't the case.
