The Kaiser Family Foundation estimates that 52 million Americans have pre-existing conditions. How many of these are in the individual insurance market? "In 2015, about 8% of the non-elderly population had individual market insurance. Over a several-year period, however, a much larger share may seek individual market coverage."
So let's say 10 percent as a conservative round number. That's 5 million people. Since Obamacare requires insurers to cover these people—and this is something Republicans can't repeal—they will still have access to coverage even if other parts of Obamacare are repealed. However, there will be no subsidies, and the price of insurance will likely be high since this population skews older. At a rough guess, probably around 3 million of these people will be unable to afford insurance.
The full disaster of an Obamacare repeal goes far beyond this, of course, but it's worth keeping this tidbit in mind. Once Obamacare's subsidies are repealed, it's likely that 3 million people with expensive pre-existing conditions will be instantly tossed out of the health care system, unable to get insurance and unable to afford proper care. And that's just the beginning.