This is a dumb question but I cannot find the answer anywhere and I am very bad with this.
My licence is about to expire and I will renew it through my the link that they have sent me to my mail. I will however change machines in about a month. Will I be able to use the licence number again on a different machine ?
Yes, that will be no problem.
You will need to remove avast from the previous system and install it on the new one.
On the new one just enter your current license and it will work.
You may have to remove your current system from your account first. https://my.avast.com
If you want to keep using avast on your current system as well as on your new one, you will need two licenses.
Either paid or free.