Trump uses the term "witch hunt" as a rhetorical device to refer to any investigations of him or the people around him. According to a Vox.com article:
Since assuming office, Trump has tweeted some variant of the phrase “WITCH HUNT!” more than 120 times in response to the Mueller investigation and critics including the “Fake News,” congressional Democrats, Hillary Clinton, various intelligence agencies, former President Obama, and “leakers” within the administration itself.
This was written a year ago, before the Mueller report was released or the current impeachment investigation. I wouldn't be surprised if his uses have more than doubled since then.
The article goes on to say
These tweets reflect the modern usage of the term — as a metaphor that delegitimizes an investigation by calling out the partisan biases and ideological motives underlying accusations of wrongdoing.
Trump's actual claims about what he's being accused of are very inconsistent. Sometimes he'll say that he didn't do it, other times he'll say that it isn't illegal or wrong. One of his spokesmen said about the Ukraine issue that it's done all the time in politics. And sometimes he just flat-out lies, like saying that the Mueller Report totally exonerated him.
The term "witch hunt" for political investigations entered the lexicon in reference to the McCarthy anti-communist hearings. Trump is trying to equate the narrow investigations into his activities to the sweeping accusations against thousands of government employees and public figures with liberal viewpoints.