We can easily order, but we have to look at the utility of the aircraft first... the Su-30 is a heavy fighter jet and we are currently looking for more medium fighters... The Su-30 can possibly do most of the things the MRCA can do, but I suspect the operating costs are much higher in case of a heavy fighter... so, the orders are being capped at 230...
Think of it as the difference between a Honda Civic and a Mercedes... A Mercedes possibly has all the features that a Honda Civic does and possibly much more, but that doesn't mean that a Mercedes can be used in place of a Civic because its servicing and operating costs are much more, as also the cost of spares...
I hope I drew a right analogy...