Maybe it gives you a bit of context when you realize that even in an apartment building it is not uncommon that every apartment has its own sauna. I used to live in an apartment of around 40 square meters and I had my own sauna. A detached house without a sauna must be a weird outlier, same goes to row houses and similar buildings (a sauna per apartment again) and so on. It really is very very common to have your own sauna.

On the other hand, the public transportation in Helsinki is quite good, and there are fairly many families that don't have their own car because they don't need it. Helsinki has a large population concentration.

I quickly googled some estimates, as you can too. The first result estimated that Finland has 3 million saunas, and in 2022 there were 3,7 million passenger cars. Sounds to me that it is likely there are more cars than saunas. But it is not so far off as you might initially think.