how many iOS versions can I update on my phone till I can not connect the 2 devices?
Theoretically, forever. Each watchOS version requires a minimum iOS version (e.g., watchOS 10 requires iOS 17 as a minimum), but it doesn’t work that way in the reverse - iOS 17 should be able to pair with any watch going back to the original Apple Watch, as will iOS 18.
That said - people do occasionally have issues when pairing doesn’t happen. But it is supposed to work that way. Currently I use an Ultra but I have my backup Series 5 still on watchOS 9 but it remains paired with my 13 Pro running iOS 17.1.1 (I had to unpair both watches when I did a factory reset and restore of my phone last month and re-paired both watches just fine).
If I was in the wedding party, I’d wear my Tag Heuer. Anything else and I’ll switch to the leather link band most of the time.