How to know the offset for filesUploadSessionFinishBatch()

You should keep track of the offset client-side, while uploading the data, so that you can specify the value when sending the next piece of the file, or when finishing the file. There's a sample implementation here.

It looks like your usage is a little different though, in that you're not using filesUploadSessionAppendV2. It sounds like your files are small enough that you can upload all of the data with a single call to filesUploadSessionStart each. In this case, that means that your "offset" would just be the length of the file data.