Detecting multiple tatties might not be so simple, if 2 were one after the other they'd show as one pulse, a bit of an improvement would be to have a pulse generator on one of the conveyor rollers and measure the travel, then if the beam is blocked say for 3 times the average length of a tattie then you know 3 have passed close together, obviously it would be sensible to count to a maximum just in case the sensors get mucked up and continuously pick something up.
Another way would be to weigh them, if you could rig something up so that they move across a plate or something you could attach a load cell to that and weigh them, maybe several at a time, weight is probably more important to you however you could just divide gross weight by the average per tattie to give you a reasonable approximation, the Hx711 module is cheap and makes it easy to weigh with an arduino, theres also lots of compatible load cells on ebay too.