Medium to reach target audience are many, which just adds to defined complexity.
1. Should I need to include article in the beginning - The medium
2. Should there be article 'a' before target - a target audience (I guess audience itself is plural so i do not need an article here
3. it should be 'which' or 'that' adds
4. There should be 'a' before defined
Thanks for your time
"Medium to reach target audience are many, which just adds to defined complexity."
There are several problems about the way this sentence [not "setenec"] is expressed. For example, I don't know what you mean by "defined" complexity.
Taking your questions one by one:
1. Since there are "many" of them, then you are talking about the plural of "Medium", which is "Media" (or, colloquially, "Mediums"). In any case, you should precede it by "The".
2. There should be an article before "target". I would use "the".
3. It should be "which".
4. If you are going to use "defined", it should be preceded by "the".
I don't know the context in which you are using this sentence, but I would express it as something like: "The many different media used to reach the target audience add to the complexity of the situation/choices to be made/environment". If I knew the context, I would try to say the same thing in much simpler terms than that.
I'm not a teacher of English, but I have spoken it for (almost) all of my life....