Each Steam key can only be activated in a single Steam account.
You can however login using the same Steam account on more than one computer, but only one at a time.
When logged into Steam all the games activated within that Steam account can be downloaded, installed and played on the PC.
Downloading the larger games onto multiple computers can take some time - for users familiar with moving and copying files between computers there are (without any implied warranty from Fanatical.com) several articles in the Steam forums and the wider internet explaining how you can copy the already downloaded and installed software from one PC to another