Ping Plotter alerts try to show the first sample that caused the alert, plus a bit more. So if you're alerting when 10 of 20 samples is over 500ms, one condition may be that those 10 happened all at once - and the alert would include those 10 samples. Another condition may be that every other sample was lost - and in this case, it would require 20 samples to cause the alert to fire - and the alert would include more data so you can see where / when the alert condition first appeared.
There is no way to fix the amount of data included in the email - as Ping Plotter changes it dynamically to show pertinent data.