Valve's product page for the Steam Deck went down numerous times following the opening of reservations for the new handheld, seemingly due to the sheer number of excited customers trying to get one of those reservations. Based on some unofficial, internal data that appears to have been inadvertently shared through the Steam store API, we may know roughly how many reservations have been made.
In just the first 30 minutes, 40,000 reservations for the 512 GB variant had already been made. This number trickled up to 52,000 over the next 30 minutes or so, with the 256 GB variant receiving roughly 26,000 reservations. Data on the 64 GB variant wasn't immediately available, but it's likely the total between all three would more than exceed 75,000 reservations in the United States alone. This number could be way higher globally, but official information isn't currently available.
The part of the website this data was retrieved from is no longer available, so, unfortunately, there isn't a way to verify this information or continue to check the latest amount of reservations made. If Valve makes this data available again, it can be retrieved via this link if you're logged into Steam in your web browser.
It appears that the earliest customers don't currently have a shipment date estimate on the Steam Deck store page. This might indicate that your order is part of the initial batch due in December of this year, but it isn't immediately clear. For those slightly later to the party, the shipment dates currently seem to vary between Q1 2022 and Q2 2022, depending on when you completed your reservation order.