how do you create a linkshell
is there a maximum of players?
is there a structure like wow guilds (officers etc??)
How do you make a custom banner?
You create a linkshell by getting one from the linkshell NPC; there's one in each adventurer's guild.
If there's a limit, I don't know what it is. And if there is one, it's probably high enough not to be a worry.
The person who made the linkshell owns it, can induct and kick members, and make and demote sack holders. Sack holders can induct and kick members. That's it for now. Supposedly linkshells are going to hook into something new called "companies" which will have a lot more tools for structuring, but none of that's implemented yet.
I'm not sure what you mean by "custom banner". If you mean the icon that displays next to your name when you're a member of a linkshell, then I'm sure it will walk you through creating one when you make the linkshell. If you mean an announcement message that a member gets on joining shell chat, that's not implemented yet, but probably will be shortly; FFXI linkshells have this feature.