What I'm trying to say is how many devices can I have under one licence?
What avast version do you have Pro/AIS/Premier ?
How many licenses do you have on your purchase/receipt.
Only what you have paid for, if your license is a single PC license then it is only one, if your license for 3PC a common multiple license, then it is good for three.
I think you are confused by mentioning devices and account as that really is unrelated to the number of PCs covered in a license. Which depends only on the type of license for have purchased. Once you install avast using your license, then you can connect that to your avast account (my.avast.com).
Of course if you are using the avast free then all of the above is moot as you can have as many versions of avast free that you need (provided they comply with the general term of use (for Home and non-commercial use).