For long backcountry mountain bike rides trying to stay lightweight as possible and with little intel on water availability (aka creek conditions):
I 1L bottle with carb mix in frame
A USWE running vest w/ 1.5L of water, I like the tight fit and stays up high on the back
https://www.uswe-sports.com/c-pace-pro-vest
Then another 750mL hard bottle in the middle pocket of the jersey which I exchange when the frame bottle is empty
And if its a really hot day with little to no water access I have two 500 ml soft bottle in the jersey or on the running vest
https://www.osprey.com/us/en/product/hydraulics-soft-flask-500-SOFTFK500.html
Also always have aqua tabs for emergencies!