Frankly : you cannot.
An Interrupt can be generated at the receive of EACH character since interrupts in PSoC 1 are related to hardware.
I would suggest to have the number of incoming characters counted (after retrieving and storing them) and when your treshold (or a pre-defined sequence) is reached you set a flag (a global variable) that you inspect in your main-loop and, if set react accordingly (or just inspect the count).
I normally use a ring buffer, all data receive would be queue up in the ring buffer and the main loop inspect the head and tail pointer.
if you know the total number of bytes is a fix number, then just do the decoding when the number of bytes received, and if that is correct then send the bytes out.