There isn't really any way to tell. Generally we quote that the vertical relative accuracy is +/- 4 GSD units. Our volume measurements have been determined to be with in .1% of RTK measured solutions.
It sounds like you have plenty of images to work with so you model is probably pretty good.
The key to getting good volume measurements is having a clear margin around the edge of the stockpile to define your reference surface with your measurement polygon. Use lots of points and it should be accurate and repeatable.
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