As many as you want. W3C has not specified limitations of usage on these tags. However as per html 4.01 documentation h1 is considered to be most important and h6 is the least important. http://www.w3.org/TR/html401/struct/global.html#h-7.5.5 would give you more insight.
You can add H1....H6 tag as many you can..there are no limit on H teg In Seo point.... and as radiant_luv says H1 has a higher value and h6 is the least value..
When you say "h1,h2... tag" does it mean h1-h6 can be used once in every page? If yes, can you please lead me to a authentic link or website which stated that?
There is no limitation for heading tags. I mostly follow w3schools but I never read about the limitation for heading tag.
there is no limitation for this tag (h1, h2 ... h6). i already try it by using loop with php script ) and it work fine
there is no limitation in using h1 tag but make sense to have very limited number per page. Google ranking algorithm gives more importance for the text in between h1 tag
+1 Focus on having one h1 tag on each page with your "favourite" keywords. If used in title, do NOT reuse in content.
I'm far from being a SEO expert, but to my knowledge it's all about H1 and H2, H4-H6 was used by spammers 5-6 years ago and today it has no value whatsoever when it comes to SEO. H3 is considered to be your content text or flow text and Google algorithms really don't care about this tag, it looks at the content (keywords + anchor text + links).
You can use as many you want, but I'll use 1 - H1 - For main site logo 1 - H2 - For the title of post H3 - It is for each subtitle I used in my post H4 - for sub-sub titles H6 - It is for side-bar headers, widget titles, e.g - Recent posts/ Popular posts etc (Unimportant things.!)
as many as you like but for SEO i like to use one H1 and maybe 3 H2's each with different content in them
There was a study done years ago on headline tags. The way it was done was to make identical pages and similar pages using different size headline tags. If I remember right H3 ranked the highest in search. I don't know of a recent study but you could do you own by using different headline sizes on similar pages. Make sure you have enough content to justify a headline. Don't just follow a headline with 2 or 3 lines of text. A least a paragraph 2 is better.