replace everybody by everyone as drafted said

Usually everybody sits on THEIR own in from of the TV.

Academically, "his or her" is 100% appropriate. However, it can be problematic to always have to include the whole phrase. In conversational/vernacular English, "his" is just fine by itself. You can also simply use "her" instead of "his", which is both grammatically correct AND shows a use of non-sexist speech; it really just depends on your views about "sexist" speech. Two alternatives, in this case, would be "their" or "one's": Often, everyone sits on their own in front of the TV. Often, everyone sits with one's self in front of the TV. The major correction is to replace "everybody" with "everyone". The rest - choosing his, her, their, one's - is up to personal preference. Hope this helps! :)

Although both of these corrections are good and valid, this is a weird sentence that I couldn't imagine a context for, because it's a generality (about everyone) and assigns an temporal adverb (often), so as a statement, it could never be "true." But even if it were, the word order is strange with "often" as the first word. So assuming this were true, I would propose: Everyone often sits in front of the TV on his/her own. However a more realistic variant would be: "Everyone sits on his/her own and watches TV at some time or another."

Often everyone sits on his or her own in front of the TV.