I can tell you individual cases of people I know about, but it doesn’t tell the whole story. Unfortunately, I couldn’t find any study that gives a definitive answer of people who lived in Ulaanbaatar and went to countryside to become nomads.
The best data I found in Statistical Office is on the migration of people from one region to another. This data is from the General Authority for State Registration. When people move from one place to another, they need to change the registration from one place to another. Registration is important as it helps you register your kids to school, go to hospitals and in general receive government service. It is not difficult to register and de-register from one place to another.
So according to these numbers, around 12,000 people moved to Ulaanbaatar and roughly 6,000 people moved back to different regions in 2019. Interestingly the Central region, which surrounds UB, has a balanced inflow and outflow of people from the region. I assumed that there should be more outflow of people from central region to UB.