Is there a name called "Stephan"? And what's the pronunciation of it? /stiven/ or /steven/?
I know there are names called Stephen and Steven, and they are mostly the same and they pronounced the same. But I heard from a friend that there is an other name call Stephan and pronounce as /steven/ or /stefen/ instead of /stiven/. I'm not sure if it's right, so could anyone from an English language country help me about it?

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You must politely ask the name bearer how to pronounce their name, it is very likely pronounced 'stefan" but it might not be. It is not unusual for people or parents to select a slightly different spelling and/or pronunciation.

The modern names we use are not always the correct names. Names like stefan are from their original respective languages as is the case for stefan.
https://nameberry.com/babyname/Stefan

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You would need to ask "Stephan" how to pronounce his name.

If I read the name, I would guess /stɛf 'an/, with the accent on the last syllable, and a definite "f" sound rather than a "v." (Steven  would be /'sti vɪn/  - accent on the first syllable).

But, really, their name sounds however they want it to sound.

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Hello, Young. Names (at least in the U.S.) can  be spelled uniquely and don't always follow strict rules. It's become quite trendy to give children names with unique or non-traditional spellings. My guess is that Stephan is pronounced 'stefen', but that is just a guess. You would have to ask the person to be certain. I hope this answer helps.

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The "ph" models are anachronistic related variants with the  "Steven" spelling but are contrastable with their realistic ambiguity and are not the same identical name. 
https://babynames.net/names/stephen