Hello tidyboy,
First you should know the kind of dasd you are working with.
For example if you are working with 3390 mode, your track will be 56664 bytes.
Second you should know the LRECL length.
If the length of your LRECL is 27998 bytes then you will be able to put two of them on one track.
If your record length is 2082 bytes then you will be able to put 21 of them on one record, but you will use only 43722 bytes on this track.
The rest is a simple calculation.
Regards,
Tzu.