There is currently no iPhone 7 and there is currently no iPhone 6s. Any iPhone 7 would be released in 2016 if there is one. The iPhone 6s is due to be released in September 2015.
The iPhone 6s will replace the iPhone 6.
Generally, the way it's worked in the past is like this: 4, 4s, 5, 5s/5c, 6/6+. Following that pattern you could expect to see the iPhone 7 AFTER the iPhone 6s.