Rafael,
1 - Then you advise me to change my projection for my map WebMecator.
No you don't have to do anything. ArcGIS Server will re-project your data to WebMercator. But if you can re-project your data than it would work faster is ArcGIS Server did not have to do it for you every time.
2 - Would any table with respect to projections WKID
I'm not sure I understand the question... The esri base maps would set the maps projection and because they are WKID 3857 all dynamic map services added to the map after them would get re-projected to WebMercator(WKID 3857)
3 - WebMecator is projection or datum?
WebMercator is the projection the datum is WGS 84