* Technically, all planets were discovered by telescope. Even though ancient civilizations knew of the existence of Mercury, Venus, Mars, Jupiter, and Saturn, they observed them with the naked eye. Telescopes were needed to reveal further details and eventually confirm them as planets.
* However, we usually think of "discovery by telescope" as finding new planets beyond our Solar System. In this case, the answer is thousands. As of October 2023, there are over 5,000 confirmed exoplanets, and this number is constantly growing as telescopes become more powerful and sophisticated.
So, the answer depends on your perspective. All planets were discovered with the help of telescopes, but we usually associate "discovery by telescope" with exoplanets.
