How many miles from home would a real estate agent show a house?

As far as you ask them to within the area they are licensed for. 

How much do they want a piece of the commission?  The last house I bought through an agent was a 90-minute drive from her office, around 80 miles.

It depends on how badly they want to make the commission.

It is up to the agent.  I know agents that will go anywhere for a client, and as a result have reciprocal licensing in many states to accommodate them.  That being said, it is in your best interest to find different agents in different areas as each agent tends to know their nearest market(s) best, and that expertise is important as a buyer or a seller.

Probably not much more than 20, as they would be less familiar with the area, etc.

It's up to the individual agent. When I was licensed, I sold property in five counties.  The farthest away I had a listing was 70 miles.  I think I'd have gone up to a hundred miles if I'd had the business.

depends on the person

A Real Estate agent can practice anywhere in the state they're licensed. Presumably how far they'd go from home depends on how much the house sells for, and how much of a commission they can make from it. If the listing is a $10 Million home, then their 3% commission would be $300,000. So, I'm guessing they'd drive a long way to sell it.