You can't check how many people directly, but you can make a sensible guess based on the behaviour. And you cannot update the database directly.
This is the procedure as I see it:
If a user finds his device is not on the database then he goes through the learning process to build the remote on his account. Once it is completed and has been installed on his Harmony remote then I suspect Logitech quietly 'add' that device to the database but marked 'unproven' maybe with a 'user number' flag set to 1. As more people install the remote, maybe refining the codes, the flag increases. Once the flag reaches a certain value (I have no idea how many they need) then they add it to the proven supported list.
So you don't upload/update the database directly, you help to update it by installing it on your Harmony Remote. Logitech do the rest automatically.
Now, if you try and install a new device on your Harmony remote then 2 things can happen:
1. The device is recognised as a supported device and installed.
2. You get the message "Device not found" and a screen asks you if you have the original remote. If you answer 'yes' then it asks for a few 'IR blasts' and tries to find a match with another device. If you say 'No' then 3 other things can happen:
a. It continues without any questions. I guess that this means a few people have installed the device but the flag is low enough that Logitech is not yet convinced it is good enough to become official. (This is what happened to me in this case)
b. It suggests an alternative model. I surmise that this means very few (1-2?) people have installed this device so it is not on the official database but Logitech can still use the codes that those people provided to suggest an alternative.
c. It bombs out saying it can't help. I assume that means no-one has tried to install the device and you are the first. So once you go through the learning process and install it then the next time someone asks for it Logitech will use option b.
As I said, a lot of this is pure conjecture on my part although the principle of users 'learned' remotes being eventually added to the database is fact. It means that Logitech don't have to add any new models to the database, it happens automatically. Pity Sling don't use the same principle.
Make sense ?
Al