read numbers AND biological significance

[lwgbioinfor]

In miRNA-Seq data set,some miRNAs represent a small total sequence reads.
In some paper,someone narrowed the number of significant miRNA by only accepting miRNAs with intensity levels >500 in miRNA chip,which are considered to be of biological significance.
In miRNA-Seq data set, How big is the mean(reads number),we can condider the miRNA is biological significance?

thanks