As far as I am aware of, you can legally carry 2 (one personal, one official)...However, it depends whether or not you claim the remaining ones. I know most of us won't claim...

I would suggest you not to carry more than 2 in your hand carry. Remaining, just put in your luggage and let them decide whatever they want to do, if they find out about too many phones. In summary, I don't think you need to worry a lot if you carry as many cellphones in your luggage. Enjoy your Nepal trip!!!