sensitivity

Nope. Although you probably don't really need to use it more than once a day. Just keep in mind that if you start to feel some discomfort and/or pain, stop all Aneros sessions until your body has time to rest and heal. It takes time for some men to re-wire and develop pleasurable sensations. I was one of those. So be patient and keep on practicing. 🙂

I don't know about less sensitive in any sort of permanent manner, but for me, if I use the aneros without much of a break between sessions, It becomes harder to feel anything from it anymore, or worse case I feel a mild-to-obvious soreness.
Taking longer (most people say at least a day of abstinence) between sessions and you'll make a lot more progress and feel it a lot more. Maybe this becomes less of an issue the more experienced you are with the device and the less sore it makes you.