Answer in brief:

• The Tirthankars were not “founders” of Jainism in the sense of starting a new religion. They are the enlightened teachers (kevalins) who revive and teach the Jain path in each cosmic era.
• They preach and model the correct path—right faith, right knowledge, and right conduct (samyak darshan, samyak jnana, samyak charitra)—so that beings can attain liberation.
• In Jain tradition there are 24 Tirthankaras in this cosmic cycle. The most recent one is Mahavira (Vardhamana), often remembered as a great teacher who organized and clarified the Jain path, but he is not the sole origin of Jainism.
• Traditions differ on certain details of the Tirthankaras’ lives and teachings between Digambar and Shwetambar communities, but both traditions agree that Tirthankaras are the supreme teachers who establish the Jain doctrine anew in each era.

If you’d like, I can point you to a JainKnowledge page that covers Tirthankaras in more depth.