What is the latest iOS version for iPhone 6s?

If its actually a 6s, current iOS version is iOS 13.6.1.

If its running iOS 12.4.8, then it cannot be a 6s, as mentioned, and is more likely an iPhone 6 or 6 plus. 12.4.8 is the latest version an iPhone 6 and 6 plus can run.

A device running iOS 12.4.8, can not be an iPhone 6S. It will be a 5S, 6, or 6 Plus.