Exactly! You should have one and only one <h1> element per page... You can have multiple <h2>s (I typically include 2-6 <h2>s on every page). It should be VERY rare that you ever use an <h3>..<h6> but if you did, you can have multiples of those as well.
If the page is offering 1 service or product then 1 h1 tag is good, but if the page is offering 2 different services and page is devided into 2 parts then 2 h1 tags are fine there. H1 means main heading on the page and if the page has 2 main services then there must be 2 main headings and so you can put 2 h1 tags.
Every page should have one but is not necessary to get good ranking in search engines this depends on many other factors.
Its fine to use multiple h1 tags, also according to the standards. It also makes sense for most websites, I.e. Including h2s above navigation would indicate that they are a part of the article, which obviously isn't the case. Many web designers have been misinformed regarding the use of multiple h1s, since a lot of people recommend only to have one on each page, which could break semantics of the page. Not having multiple h1s wont harm your page, but we should always aim for semantics, and having multiple h1s is semantically correct, just like having multiple h2s would be at times. Just DONT use h1s around everything, simply for the sake of optimizing your site. Such practices are very likely to be picked up as spam, and as such any keywords are likely to be discounted. Because its so easy to detect! Beware, you should only use multiple h1s when it makes semantic sense to do so, having multiple h1s is valid according to standards.
According to the SEOs opinion only one bcz each page contains one main heading and ca nse many sub, subsub etc...
That would depend on which SEO you ask. While having multiple h1s can be considered semantically correct in some cases, including them wont hurt your rankings, unless you use them in suspicious ways, for example for keyword spamming.
It depends on number of topics on 1 web-page. H1 Tag is use to highlight the importance of certain text on a web page usually Headings are in H1 Tag. So, if you were to have a single page with 2 concepts, you must have put 2 H1 Tags on it. Just don't spam it,,,