Even if this is a propaganda number and the true number is only half or one-third of 28,000 that’s still a huge number compared to the size of Ukraine.
Mostly foreigners though.
Ukraine is 43 million people. If half are male, and only 10% were military eligible, thats still 2 million capable defenders.
Battlefield surveillance is good, thanks to drones. Then add in intercepted communications and other sources.
The daily “ clobber count” of dead/WIA lost in battles as briefed by the Russian MoD is sometimes 1,000 or more per day. The average is given at 750.
There are 30 days in a month, so do the math.