Most of us aware of it , that the no. of tabs on a form is not unlimited.(If not aware keep it in mind).The limitation is 8 tabs max.
So if at any instance there is requirement of more than 8 tabs(index actually started from 0-7), then we need to make our fingure bend(even if its going against and risky).
Just need to be changed a variable in a file as follows.
find "formeditor.aspx" from the Tools\FormEditor of the installed CRM instance.
change it to your desire value.
save and close and reset IIS.
there you go.