There is two sides:
Ukrainian government could exaggerate numbers, but it is the only source of truth, I mean each combat battalion reports eliminated targets of their mission, which forms daily report of killed personnel and destroyed vehicles. In addition to this there is x3 of the killed personnel gets wounded and unable to continue serving it's army, means that according to today (Aug 12) report of Ukraine of 43200 liquidated personnel there is also roughtly 120000 wounded, the rates are usually confirmed by analysts and statistics from other wars or conflicts.
At the same time there is other independent sources which analyses destroyed vehicles and liquidated personnel using open sources which is huge limiter during active conflict.
The most accurate report could only be made after the war ends, when all the data will be available and independent services will be able to calculate everything properly, and even here could be disagreements.