There are officially 9 games in the main Ys series with several older games being replaced with newer up to date versions.
The current timeline for the events of the series is as follows:
1. Ys Origin - Takes place about 700 years before the start of the series
2. Ys I & II - The start of the series
8. Ys Seven - Takes place about 6 months after Ys VI as Geis mentioned it's been about half a year since he last saw Adol.