According to Vadym Skibitskyi, a representative of the Defence Intelligence of Ukraine, the Russian grouping on the border with Kharkiv currently has 35,000 troops, but is likely to increase to 50,000-70,000.
Russia is also “creating a division of reserves” (i.e., from 15,000 to 20,00), which it can add to the main forces.
Even so, according to Skibitskyi, the Russians do not have enough resources to capture a large city. The Economist notes that Western military officials also share this opinion.