Depends on the situation. In some cases, none at all. In others, no amount is great enough to do the job.
In between, we have situations like the Korean Peninsula, where the spread of communism was halted through outright warfare, and could possibly have been rolled back further up the peninsula to the Korea-China border with a greater application of force. Similarly in Vietnam. The North Vietnamese Army (communist, intent on establishing a PRC-backed communist hegemony in the region) was almost entirely used up during the spectacular but disastrous Tet Offensive. A greater application of force at that time, rather than a systematic retreat and withdrawal, would have almost certainly resulted in decisive defeat for the communist forces in Vietnam.
Then, of course, there's the case of Eastern Europe at the end of World War 2 and during the Cold War. Had the western allies been willing to apply force against the Soviet Union immediately following the fall of Nazi Germany, the Eastern European nations might have peacefully elected socialist governments, but certainly would not have suffered for two generations under the tyrannical and inhumane Stalinist regime of the Soviet Union.
And let's not forget the FARC. The Colombian people don't seem to be in any hurry to vote themselves a communist regime. Not being an expert in such matters, I can't give you an exact amount of force would be necessary to root the rebels out of their mountain camps, but I'm sure it's finite, and well within the resources of any one of several powers in the Americas.