I'm playing with 2 planes and likely medium world size (whatever is default).
I just took my 16th city and the penalties started. 16 cities covers a tiny fraction of the land. I instantly went from having a solid economy to having to think about things. Penalties at this fraction of land coverage is punitive.

I second the suggestion I saw elsewhere to have the option to raze your own cities.

BTW, the banner still says the penalties start after 10 cities, it would be nice if the info banner were accurate.