lets take all the node contain maximum no of keys which can be hold by them , now insert a key in the leaf node then the node will break in two node and transfer one of the key to the parent
but we know that the parent also have maximum keys so adding one key to them will split the node in to two node and one key will be transfer to the its parent ,again same thing happen
at the end when the key will be inserted in the root node the root node will break and create a new parent which will contain only single key
so one level has been increased
so total level is 7