The question of using Multiple H1's is apparently still relevant, as I often see SEOs spreading stupid misinformation, such as, saying you should only use one h1 per page. This is not accurate however.
In the real world, you would often use more than a single h1. It is about semantics. Using multiple h1 elements is semantically correct when there is logical reason to do it.
An example would be to use a h1 for your main content, and another for your navigation menu. Using more than one h1 in this way explicitly places your navigation on the same heading level as your main content.
Last time I covered this subject, I got more in-dept about the technicalities of HTML5 sections, you can read this article here: How You Should Use Headings in HTML
Why use multiple h1s
Had you instead used a h2 for your navigation, in all likelihood, what you would in fact be saying is: "this navigation belongs to the main content", which could be confusing to people with screen readers.
Semantically, the navigation would usually fit best in a separate column on a site, optionally below its own h1 heading.
Now, some are also in the habit of using an h1 for the logo or the name of the website. While I would not do this myself, it still would not change things. Search engines are perfectly able to tell apart your main content from your navigation and your logo without using different headings.
More than one h1 and SEO
The main concern in regards to having more than one h1 has to do with how search engines will treat a site. Rest assured it has no negative effect. I have tested this multiple times on different websites, and none of them suffered any negative side effects from having multiple h1s.
I even tested exclusively using HTML5 section + h1, and this also did nothing to hurt SEO. There are other reasons why you would avoid that, however, mainly lack of support from accessibility technology.
If you want to learn more about the technical considerations, then you should read the article i mentioned earlier, as it is still relevant in 2020.
See also: How You Should Use Headings in HTML
The standards are clear
It was never prohibited to use multiple h1s, and in my time, I have never had any problems with it. You will be surprised how good some of the worst coded sites are doing in the SERPs, and many of these sites will be using a lot of invalid HTML.
Now, if such websites can rank well, why would you think that a perfectly valid practice should be harmful? It makes no sense. Fact is, the validity of your code has little influence on SEO.
That does not mean we should not be careful, it just means that using multiple h1s is not one of the things that is going to matter. The sooner SEOs stop spreading such misinformation the better, because sometimes it might actually hurt the accessibility of people's sites.