Take a look at description of Serial.readBytesUntil(character, buffer, length).
Then try this PSEUDOCODE
char MyBuffer[] double array to store packets
int i = 0; starting index to array
while (Serial.available()) while loop when Serial buffer has data
{
read a packet of characters from Serial buffer until character ',' and writes it into MyBuffer , returns count of characters read in packet
Character Count = Serial.readBytesUntil(character, MyBuffer*, length); removes packet read from Serial buffer*
i++; read next packet
}
where
character = ',' parsing character
buffer = MyBuffer l
length > then max length of packet expected
PS ignore capital I , it should be lower case - autocorrect at its best