I cannot see invoice numbers and there is no mention of automatic numbers in the default form
Manager version? Operating System? Screenshot? I noticed this is your first post and I would advise you to thoroughly read FAQ - Manager Forum which includes the information you need to provide for others to help you on the forum after exploring the guides and searching the forum if a similar topic and discussion exist.
It would also help to know where in the program you cannot see invoice numbers, which type of invoice you are referring to, which form default page you are viewing, etc.