1. Catalina y Julio saben regatear.
This sentence means that - Catalina and Julio know how to bargain. You should use saber here instead of conocer because the verb saber is used to denote that you have the knowledge of doing something, whereas conocer means that you know somebody, or some place well.
2. Mi novia conoce a mis padres.
This sentence means - My girlfriend knows my parents. So obviously here you would use the verb 'conocer' because that means that you have met that particular person before, as is the case in this sentence above.
3. Yo conozco Madrid.
This sentence means - I know Madrid. Know, in the sense that you are familiar with Madrid because you've been there before and wouldn't get lost there easily. This is why you use conocer instead of saber here.
4. Sandra y Andrea conocen la universidad.
The same thing is here as in sentence 3 - this sentence means - Sandra and Andrea know the university. They know it in a way that they know where is what, and where to go, as they've been attending lessons there for a long time now.
5. Yo se montar a caballo.
This sentence means - I know how to mount a horse. You would use 'saber' here because you are demonstrating the knowledge of a particular skill, in this case, mounting a horse. So conocer wouldn't really fit in this sentence.
