There is no maximum for objects in a sector.
However there can only be a total of 512 objects in the level.
Also you can only load up 64 each of PODs, FMEs, and WAXs.

So you could have 1 FME and 512 objects that use that FME.

Notes
This limitation has probably been removed, because you can use more than 512 objects in a level and all work well.

On the other hand, there is a limit to the number of objects that can be displayed (or active ?) at a given time. When you reach it objects begin to flicker in and out, enemies don't appear but do fire at you, etc. It begins at around 500 objects too…