You can do it say in a cavity slide too only make the medium a little viscous which will make the worms sluggish in motion.

To count worms - just dilute your sample and then count the number using a stereo microscope and then extrapolate the number of your original sample. 

As for preparation of C. elegans lysate for subsequent HPLC analysis of your drugs: harvest C. elegans by spinning down the nematodes sample - flash-freeze the same in liquid nitrogen. Then grind the frozen pellet and resisted the lysate in a buffer of your choice. 

You can use a high resolution scanner to scan petri dishes.