grammar - Does sentence(s) with semicolon count for one sentence?

I am just wondering if I merge two sentences using a semicolon, then are they become one sentence or they are still two sentences but more closely connected?

For example, is the following sentence(s) count for one sentence?

He knows how important attending lectures is; however, he still missed today's lecture.

Edit

As some comments pointed out, the question seems kind of pointless without any context.

I am doing my logic course homework, which requires me to translate English into logical expression. We usually count each sentence as one premise or conclusion. I encountered one problem containing a semicolon so I am not sure whether I should divide them into two premises or just leave it as one.

I guess it's better to ask my professor.