The workshop manual says that the yamaha XT/TT 350 engine holds 1.3 litres (1.37 qt) for periodic oil and oil filter change, and hold 1.6 litres (1.7 qt) when first filling the engine from a rebuild etc..
Sign up at myyamaha.com and download the service data sheet for your bike which lists the oil type and capacity. https://www.yamaha-motor.com/sport/myyamaha/login/L3Nwb3J0L215eWFtYWhhL2hvbWUuYXNweA==/starthere.
When changing the oil in a Yamaha Virago 250, it depends on the engine size as to how much oil to put in. If it is a 1.4L only 0.36 gallons of oil are needed. However, if it is a 2L then 0.52 gallons are necessary. Do not overflow the reservoir.
Most Yamah Enduros have the oil amount stamped into the case right next to the fill/dipstick cap. It will usually be listed in cc. For instance, my 1978 GT80MXE says 500cc. You could also call your local Yamaha dealer and ask them. Be sure NOT to overfill the gearcase!