How many devices can I have under one account

Can I have more than one device under the Standard Avast account?

You can have as many as you have protected by avast. 8)
FYI my avast account has my laptop as two devices. One “free” and the other with an AIS license.

What I’m trying to say is how many devices can I have under one licence?

Depends on the license. If it’s a paid license there are limitations per the license whether the license is for one, two or three machines.
If you are asking about the “free” license, then it’s one computer per license/registration. No matter the number of computers you
have running the free version, each must be registered separately. If the machines are in a business environment then they must
have the proper business license. Schools and certain charities are exempt from the need of a business license.

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).