Hi! To my knowledge you cannot directly stain axons using chemical dyes. Nevertheless you could think of staining the myelin sheath with the Kluver-Barrera method and therefore you could indirectly know how many axons or their diameter for example.
You can label the ais with an anti-neurofascin antibody (neuromab, Cat #75–172) followed by a secondary antibody in live neurons. It works very well in rat and human (but not mouse) neurons...