The demand for health care is not infinite; it is quantifiable. Some people need very little health care: they go through life with scarcely a day off and end up "dying on the golf course". Others are less fortunate and need varying amounts. It is thus possible to ascertain the average and multiply that by the total number of people.
With a low-taxation policy, the demand will never be fully met. With a high-taxation policy it might be. But it might be argued that the need to improve other things - education, for example, should come first.