I think Prp's limit is about 8000 faces per object. This may sound low, but that is a GOOD thing. You should get into the habit of making your objects as low poly as possible, and this "limit" acts as a timely reminder. It can be easy to "over-poly" your objects because you want them to be super smooth, or super detailed. The reality is, that most "detail" is done with texturing, NOT modelling, and that most objects are "smoothed" by Uru anyway, so all that smoothing is just unnecessary (I have simplified this, for arguments sake),
Basically, your biggest single object is likely to be your landscape, and, if your landscape is quite large (in releation to your avatar) then it will probably need to be make out of different sections. But dont be misled into thinking that the landscape's model needs to be very detailed .
... take a look at Sonavio - one of my Ages available on the ULM & Drizzle - that complete landacape is made up of just TWO objects ... each one is only 7, 500 faces each.
(You may be surprised when you finally get your landscape exported and into Uru just how large you can make each of those faces without it being too "jagged" in Uru. The real trick is in the texturing ...

). In any event, Uru would just not cope with objects with 10's of thousands of polygons PER object. To give you some idea about polygon usage, the WHOLE of Relto .... everything ... all those bits and pieces, trees, rocks, plants, islands, etc ... comes to a TOTAL polygon count of about 100,000 (IIRC).
(this means your one object has almost as many polygons as the whole of Relto)
when it comes to Age creation ... "DOH" seems to be my middle name...