Hi Mohammad,
Defaulting the number range can be achieved by using the badi BADI_CRM_BP_UIU_DEFAULTS.
You have to create a two different number range groups and in the badi write a logic to default the field BP_GROUP based on the BP_CATEGORY field.
For individual account BP_CATEGORY value will be 1 and for corporate account BP_CATEGORY value will be 2.
Thanks & Regards,
Vignesh