Idon’t think so.I think Avast also sell its licenses for 1 PC.You can actually choose it.Go to Avast!official website and click the image of Avast Pro or Avast internet security.If you choose one of them,you will see a small fonts there:“Add computers or years”.Just click it then you just need to change the quantity you want to buy.
I used to use a registration number on earlier free versions, but this doesn’t seem to be the case with the current free version. If you register from within the program it seems that’s all you need to do. Is that correct?