Hi Vajda @Carol ,
Not sure if I understand you correctly...
If you want 250 Frames, you set the Frame Range from 1 to 250 in the timeline or in the Output Tab (they're the same):
And then make sure that Frame 251 is the same as Frame 1.
So, it won't reach Frame 251, but instead of Frame 251 it plays Frame 1 after Frame 250. But as these are the same, the video loops seamlessly.
Yes, it's all about making sure that whatever comes after the last frame, has to be exactly the same as the first frame. So if your range is from 1 to 10, then fame 11 must be exactly the same as frame one in order to loop. If your range is from 1 to 500, then frame 501 has to be the same as frame 1. That is because if the time line ends at say frame 500, it will go back to frame 1, so the so it has to feel as if it just continued to frame 501.