As a rule of thumb, it takes about ten times as many men to fight a guerrilla force successfully as the guerrilla force itself comprises. 30 German divisions -- 300,000 to 400,000 men -- were insufficient to maintain control of Yugoslavia in the 1940s, as they would be today.
Today, there are ten or eleven million Yugoslavians, most of whom, it can be assumed, could be induced to support guerrilla warfare against NATO. Another rule of thumb of guerrilla warfare is that a supportive population can put about ten per cent of its population into direct-support activities and about one per cent into full-time combat. That is, Yugoslavia could maintain a 100,000-man guerrilla army plus an active support structure, the suppression of which would require a 1,000,000-man army of occupation, for several years (until the entire culture which supported the guerrillas was forcibly eradicated, presumably at a rather high price).
The 200,000-man figure is probably another vacuous fiction, similar to the widely-noised assertion that bombing would force Milosevic and company to yield Kosovo to NATO.
None of this takes into consideration the possibility that continued NATO activities in the Balkans will cause a resumption of active paranoid nationalism in Russia and Ukraine, which would lead to other difficulties possibly requiring further armed forces.