Hi Kuat and welcome to the forums!

the number and make-up of your multi-boxing group is up to you... but... the complexity of the interaction between you and all your characters increases with the number of different classes. if they are all the same, then a single keyboard layout for all is fairly easy. if you made a 5 man group with 3 different classes (ie: 3 dps mages, 1 tank warrior and a priest)... then you would have to figure out the keyboard layout that would allow you to effectively operate all 3 classes at the same time... unless you can easily operate 3 keyboards, the combined keyboard can be challenging to figure out

i hope that helps.