The Steam client is the only way to activate a Steam product key , and they only allow you to enter one product key at a time.
With some Humble Bundles, the keys are actually for a package of games, and not given individually, although the current bundle (Humble Bundle with Android 5) does not seem to give keys in this fashion.
You could potentially automate the process with some 3rd party app, although you'd have to be aware that the key authentication process can take a variable amount of time.
Also, the games default to being installed immediately, which might not be what you want.
gaming.stackexchange.com/users/13845/agent86
There is no official way to do this, but I wrote a simple script for Mac, that does exactly that: https://github.com/timothybasanov/steamkeysactivator Script uses AppleScript and could not be ported to Windows or Linux.
gaming.stackexchange.com/users/50678/Timothy Basanov
For Windows, there's now the Steam Bulk Key Activator , which will read a text file of keys and plug them all in for you.
You'd have to copy the keys out of the webpage yourself, but it still saves the tediousness of the Steam aspect of it.
gaming.stackexchange.com/users/11015/Bobson
SteamBulkKeyActivatorScript - Right Click, Save As.
Run with AHK
For windows only, uses Autohotkey(AHK) to automate the steam interactions.
I've fixed the problems in it so far.
You must download the AHK source .
The exe's aren't compiled with the latest version.
You will need to install Autohotkey to make it run from the source.
I just tested it with a dozen keys and as of this posts modified date, it's fully functional.
Credit to original author
You can use chocolatey to install AHK as well (at least after you've installed chocolatey).
choco install autohotkey
gaming.stackexchange.com/users/6419/Vijay
Register Key(s)
Page
The Steam Register Key page (available only if you're logged in) offers both an input field to enter a single key and an "Activate Multiple Products" button, opening a new input field in which you can enter multiple keys, one per line.
Single-Key Auto-fill
The key input field can be auto-filled by by appending a key= parameter to the URL, e.g., https://store.steampowered.com/account/registerkey?key=XXXXX-XXXXX-XXXXX
However, this is not entirly automatic; the user will still need to click the "Continue" button in the page brough up by this URL.
If you're redeeming Humble Bundle keys, exposing the key now shows both the key and a button that will open the above URL in a new tab, so you need only click that and "Continue" for each key.
Multi-key Auto-fill
The above URL can also be used for multiple product codes, though it's not quite as user-friendly.
Concatenate the multiple keys in the key= parameter with a comma between them, e.g. key=XXXXX-XXXXX-XXXXX,YYYYY-YYYYY-YYYY .
This will fill the single-key input box with the comma-separated keys.
This cannot be accepted as a submission ("Continue" will produce an error), but if you then push the "Activate Multiple Products" button you'll find the full contents of that field in the multi-line input box with each value on its own line, and you can submit that in the normal way by clicking "Activate Products."
gaming.stackexchange.com/users/124847/cjs
No, there is currently no way to activate more than 1 product key at a time with Steam.
gaming.stackexchange.com/users/19827/kotekzot
After purchasing the current Humble Bundle, I wondered if it was possible to activate all of the 6 games in Steam at once, without having to do a "Product Activation" for every game.  I could probably make a script, which does this automatically, however I was wondering if there already exists a way of doing it.
Bulk activate product keys in Steam
gaming.stackexchange.com
<steam>
gaming.stackexchange.com/questions/107580
gaming.stackexchange.com/users/10411/Tyilo
The Steam client is the only way to activate a product key. For Windows, you can now ge the Steam Bulk Key Activator and the Steam Register Key page that opens a new input field. While the client version of Steam does not offer the option to activate multiple keys at once, there are few options that can help with this task, such as third party softwares.
The community points that there isn't any option to bulk activate product keys in Steam. Third party software usage is adviced. A user is stating that accessing Steam activation page via browser, will do the trick.