Quality is much more important than quantity. Look into public transportation. Talk to teachers and other school officials about these and other options. Many businesses allow time for employees to volunteer in schools. Another possibility is to host a class field trip at your place of work.
Some parents have health conditions and are unable to volunteer at school.
But they can still help. For example, you could offer to do telephone work from your home. If you call ahead of time, the school often has a place where your children can stay while you volunteer or attend PTO meetings. Remember, by volunteering, you help all children, parents and school staff. A. Not at all.