Not all. When it comes to spread, hospitalizations & deaths are a pretty good proxy for positive tests. They're just a lagging indicator. But yeah, no one knows how many people actually have the virus.
Fun/morbid data for the US:
Average flu season deaths, last 3 years: ~44,500
Average car crash deaths per year: ~38,000
Average heart attack deaths per year: ~647,000
Current Covid-19 death toll: ~12,700
Projected Covid-19 deaths: ~60,000, given current level of physical distancing
I've sorta given up arguing ITT, since it's become more of an idiot test than a discussion.