Take a look at the published literature for the number of worms needed, but if I recall correctly, you need data for at least 100 worms that you find dead of old age. Worms that die of other causes (bag of worms, gonadal protrusion, etc.) or just go missing don't count.

I've done some lifespan assays using the standard ~5 cm diameter petri plates. You can put 10-15 worms per plate. The first few days are the hardest because you have to transfer them to new plates every day to separate them from their larvae.