Is there a maximum number of skills individual users can add to their profiles? And if so, what is the default number and is it adjustable in system properties?
Also, if I search for particular keywords for a skill, do people who are endorsed more for that skill appear higher up in the list than people who have been endorsed fewer times?
Yes, endorsements add weight to the search results. If you have 5 people endorsing you for community management and I have just two, you will come up higher in the results when one searches for community management. I don't think there is a limit (at least that is configurable in the admin console). They are essentially tags and in practice there are no limits on tags.