Short answer your witnesses may be anyone who is 18 or older.
You will require 2 to sign along with you both and your Celebrant on the day on 3 documents. Including the Marriage Register, Official Certificate of Marriage and your Keepsake Certificate.
Your Celebrant will ask then by name to come up for the signing so they need not worry.
Some choose from their bridal party, but because they already have an important role some couples prefer to choose someone they could not have in their bridal party like a sibling or another close friend.
Last year some of my couples chose both of their Mums which was really special.
Entirely up to you, sometimes if it is an elopement your Celebrant can assist in finding witnesses for you.
It is good to have your witnesses sorted in advance if possible that way you can pass on their full names to your Celebrant ready for the legal paperwork.
Hope this advice helps you with your decision 💜