suppose i order 10 SBIN shares as BO order but only 4 filled and 6 are pending.Now for modifying price what quantity i have to mention 6 or 10 in order_modify call
You need to mention 10 in the order quantity. You always send the same quantity you placed unless you want to change the quantity it self. In this case to modify quantity for all the pending quantity you send quantity 10. Suppose you want to increase the quantity by 1 then you should 11 now as quantity and since 6 are already pending the pending quantity becomes 7 now.
open
only butfilled_quantity
andpending_quantity
might differ. So you can edit the open order as usual - https://kite.trade/docs/connect/v1/#modifying-orders