Here’s what I found.
Short answer: 252 (April, 2021). 32 of which are “obsolete and deprecated.” So, 220.
Long answer: trick question, because HTML5 supports custom elements.
Longer answer: entire list here.
HTML: Hypertext Markup Language provides the structure of the page.²
(HTML) Elements: The elements of the language label pieces of content such as “paragraph,” “list,” “table,” and so on.³
A little Python and a lot of curiosity can go a long way: