It is not entirely clear which limit you are looking for because the "usage of maps" can follow into a number of different map API categories. For just displaying maps, you can display unlimited maps without usage limits on moblie apps using the Google Maps Android API v2 -- the supporting documentation comes from the FAQ.
The Maps API usage limits apply only to the following Maps API
services:
Google Maps JavaScript API v2
Google Maps JavaScript API v3
Google Maps API for Flash
Google Maps Image APIs:
Google Static Maps API
Google Street View Image API
Note that Google Maps Android API v2 does not appear on the list, and thus, is not subject to usage limits.
However, your question seems to be asking about routing, not maps. If you need routes, you need to use the Google Directions API, which is not part of Google Maps Android API v2, and subject to its own usage limits. Specifically, 2500 requests per key per day. Since directions is not tied to mobile app, it has no notion of a device.
In summary, if you want to display a map with a route you have generated on your own, there are no limits. If you need Google to give you the route, you can only do 2500 requests for routes per day. If you need more, you have to get a Google Maps API for Business account or find another routing solution.