Yes, that is one of the basic rules for using は and が taught in Course 1. However, when using あります, there is also a rule that が is used by default to mark the item that exists or is had (Course 2 Lesson 1 Verb Usage).
チケットが あります。(CHIKETTO GA ARIMASU) = There is a ticket. / There are tickets.
And particles are not necessary after counters such as こ (for round objects)、ほん (for long objects) and まい (for thin/flat objects). → Course 1 Lesson 11
チケットが さんまい、 あります。(CHIKETTO GA SAN-MAI ARIMASU) = There are three tickets. / I have three tickets.
なんまい (NAN-MAI/how many) is a question word, but it is a counter. Therefore, there is no particle after it.
チケットが なんまい、 ありますか。(CHIKETTO GA NAN-MAI ARIMASUKA) = How many tickets do you have? / How many tickets are there?
When talking about a specific item, は is used instead.
チケットは ありますか。(CHIKETTO WA ARIMASUKA) = Do you have “the” ticket?
There are so many rules, aren’t there? Reading and practicing with Japanese speakers or learners will help. Keep up the good work :)