How does the number of substitutes affect the price elasticity of demand for a product or resource?   What does a higher number of substitutes mean for the slope of the demand curve?   What does a smaller number of substitutes mean for the slope of the demand curve?

Use only the attached Chapter 7 and Special Topic 6 for your references.
How does the number of substitutes affect the price elasticity of demand for a product or resource?  
What does a higher number of substitutes mean for the slope of the demand curve?  
What does a smaller number of substitutes mean for the slope of the demand curve?