Bhoke,
Welcome to JOF!
Depends on RV size, floor plan, and plumbing layout requirements. I use a little under 2 gallons by the time I finish filling the P-traps in my Jay Flight 24RBS.
With your 16XRB I would purchase 2 gallons...., save the leftovers for next year.
Don't forget to properly position the HWH by-pass valves 'prior' to winterizing with antifreeze......., and put a cup of antifreeze in the toilet bowl to keep the seal lubricated.
Bob
__________________
2016 GM 2500HD 6.0L/4.10
2018 Jay Flight 24RBS
2002 GM 2500HD 6.0L/4:10 (retired)
2005 Jayco Eagle 278FBS (retired)
1999 Jayco Eagle 246FB (retired)
Reese HP Dual Cam (Strait-Line)