do i need cable tv service to get live tv on apple tv?

Internet access, yes, and where required a subscription.

This is explained at the page for Apple TV: Apple TV 4K - Apple

"Live TV. Apple TV 4K is all you need to stream live TV from the world’s biggest and best networks, broadcasters, and pay TV providers. Watch sports from ESPN and MLB. Catch up with news from ABC, CNN, and Bloomberg. And kick back with shows on Hulu, YouTube TV, and Sling TV."

To make sure you understand, the Apple TV does not have an antenna based tuner so you cannot receive broadcast TV channels over the air. Any live TV would have to be through a subscription to any number of streaming services as pointed out by user David McKinlay. A traditional cable tv subscription is not required.