how to use old and new smallRNAseq data sets for data analysis

I have some smallRNAseq data (already sequenced) and would like to generate smallRNAseq from another few samples. but at the end I would like to include both new and old batches in the data analysis.
For smallRNA we know that the libraries can be quite variable (even between samples in the same library prep), and also we want to use both batches in our analysis. so the question is that how many samples (or percentage) must be repeated between a old project and new project to be able to include samples from both batches in our data analysis?