How many worms do you put in a plant to help it grow faster?
Worms aren't needed for plant growth. Worms are one of many species (most of them microscopic) in soil that can provide benefits for plants. Worms are involved in the cycling of nutrients to plant available forms. They also tunnel pathways that can be used by water, air and root growth. An exact number of worms that be ideal for making soil the most hospitible for plants is hard to say.
A more practical way of looking at it is creating an environment that is inviting to worms. Organic matter in soil is an important food source that will attract worms and other benficial soil organisms. Ideally a garden soil should have 5% or more organic matter, but that sn't always easy to achieve depending on the loca conditions. Organic matter must be reguarly maintained to keep soil organisms happy.
The other bit about worms in your soil is that you want to till as little as possible. Would you want to live in a neighborhood that is getting destroyed regularly?
Tilling to worms is like a catostrophic earthquake to humans.