nigelbleddfa1,
keep in mind that the number of horses can change because of withdrawn horses. In rare cases, the number of horses can even increase, if a previously withdrawn horse is re-instated again. 
You can use variable "runner_number" to get market's number of runners. 
If it is easy to join your 2 trigger files into one depends on the difference of the two files. If your "over 8" and "under 8" versions are similar, you can add a coundition like
"and trigger expression runner_numer is greater than 8" 
If the 2 versions of your trigger file differs only by some parameters, 
use this expression part to increase a parameter value for the "over 8" version by 2
<some expression part for "under 8" version> + IF (runner_numer>8, 2, 0 )
If you 2 versions are completely different, you can add 
"and trigger expression runner_numer is greater than 8" 
or 
"and trigger expression runner_numer is less or equal than 8" 
to all trigger conditions and merge your 2 trigger files.
Martin