How many images should you shoot for star trails? 

As I mentioned before, anywhere between 100 and 500 images will do. However, if for some reason the camera didn’t reach 100 images that can also work, only the effect will be a little different.
The free version of LRTimelapse allows processing of 400 images only. To bypass that, you can make two shorter videos and combine them in free video editing software such as Windows Movie Maker.