Will my paper get an ISBN/ISSN number if I upload to arXiv.org?
No. ISBNs are assigned to published books (print or electronic), and ISSNs are assigned to published journals (or other serials, such as conference proceedings). 

arXiv is not publishing either of these and so will not issue such identifiers.

This is effectively saying "if it's been published, it's not allowed, but informal circulation online or as a working paper is fine". Having an ISBN or ISSN is a simple (though of course not perfect) test to see if something counts as "properly" published. (It's worth noting that papers as such never get an ISSN - the serials they are published in have ISSNs but this will be common to all papers in that serial.)