Game ready model means both low poly and efficient uv mapping and textures.
With poly count, that's down to the engine you are using and the type of model you are making eg - character, environment, vehicle prop etc. In general the lower poly all of your assets the less power needed to process the geometry which will make it more playable on lower end machines. In most cases for characters weapons vehicles etc they are rigged and animated and more polys may be needed to smooth of the mesh around the joints which will save tearing.
The other big part to the model is the texture and your uv map. Make good use of your uv to get as much detail on your model as possible and don't forget normal maps and spec maps will bring the model to life. The bigger the texture the more power that is needed and graphics memory, so make your textures as small as you can for low priority and smaller models and ramp up the size for more detailed stuff like characters etc.