https://support.steampowered.com/kb_article.php?ref=8360-WEJC-2625#multiple
How can I purchase multiple copies of the game with one credit card?
In order to deter fraud, Steam does not allow multiple purchases of the same product with the same credit card.
To purchase multiple copies of a game you must attempt a purchase (a "test purchase") on each Steam account before we can remove the lock on your card. Simply enter your credit card information in the Steam Account you wish to make the purchase on.
Once you have placed a test purchase on each of the accounts and received notice that it was declined, please contact our billing department and provide the following information:
The last four digits of the credit card you are using for your purchase.
The Steam Account name(s) you would like to purchase software for.
The Steam Account name(s) you have purchased games on prior to this attempt.
We will send you confirmation after reviewing the issue and enabling your card for an additional purchase.