"I am interested in weighing whether Trump's recent actions are unusual, or if they are merely getting more attention than such actions typically receive"
Addressing this part of the question, since the initial formulation asking "how many" is largely unanswerable, and since this particular approach has never, ever been used before in U.S. history.
Federal employees do not have clearances revoked without cause. Typically there are financial irregularities (debt, gambling problems, unusual purchases or sudden influxes of cash), or intelligence about communications or unexplained travel that might suggest a national security concern. They might be suspended pending investigation or revoked entirely for improper/deliberate mishandling of classified information.
There are procedures in place for due process, and established reasons for revoking a clearance for cause or national security, none of which have been followed so far in this situation.
From The Washington Post:
"Trump’s letter appears not to have actually formally revoked anything. Rather, the president said he would “direct appropriate staff of the National Security Council to make the necessary arrangements with the appropriate agencies to implement this determination.” "
Under Executive Order 12968 the heads of agencies are responsible for granting or revoking clearance, and for establishing and following procedures to do so. In the absence of a compelling national security interest (and there is not one here), the EO requires them to follow due process and those policies. So, in the unlikely event we could get an accurate count of "how many" times high-level officials have lost clearances, they would almost all be for legitimate causes backed by investigation and evidence.
This NYT article about David Petraeus, mentions former CIA director John Deutch, who lost his clearances for keeping classified information at home. Petraeus' lover Paula Broadwell was an Army reservist who lost her clearance as a result of the affair, but I'm not actually clear whether Petraeus lost his.
Mike Flynn, the short-lived Trump National Security Advisor who pled guilty to lying to the FBI & kicked off this whole debacle might not have even had his clearance officially revoked yet, and he was accused of plotting kidnappings and conducting illegal negotiations with hostile foreign actors...
Trump, in an interview with the Wall St. Journal (paywall) explicitly said his move to revoke clearances was a direct response to the Russia investigation, which he believes undermines his legitimacy:
"“I call it the rigged witch hunt, [it] is a sham,” Mr. Trump said in an interview. “And these people led it!” He added: “So I think it’s something that had to be done.”
He also said this action was based on his personal opinions, and did not offer any proof or rational justification for his actions:
“I don’t trust many of those people on that list,” Mr. Trump said in the interview. “I think that they’re very duplicitous. I think they’re not good people.”
Ironically, he based the action in part on Brennan's TV & online statements, indicating that this is a personal, political, and likely unconstitutional violation of the First Amendment aimed at a person who said things he didn't like:
In a statement that accompanied the announcement to revoke the security clearance, Mr. Trump cited what he called Mr. Brennan’s “wild outbursts on the internet and television” as reasons for the move.
As to how often this has happened before & whether it's atypical:
“I cannot remember a time when the president of the United States got personally involved in the status of individual security clearances within the country,” said Larry Pfeiffer, a former chief of staff at the CIA.
No, the POTUS has never before in the history of the current U.S. security clearance system personally & publicly stepped in to revoke the security clearance of high-level officials, past or present, for what he has directly admitted are personal, petty, and arguably criminal, unconstitutional reasons.
For a counter-example of the next closest thing, Brent Scowcroft - former National Security Advisor under Reagan & Bush Sr. - published an op-ed in the run-up to G.W. Bush's invasion of Iraq that opposed toppling Saddam Hussein. He was already "on the outs" with the White House at the time, and subsequently did not have his position renewed on a Foreign Intelligence advisory board, but there's no indication he was publicly lambasted by GWB in the media or actively had his clearance revoked. He certainly wasn't called "duplicitous" or "not a good person" by the President.
Since this statement may not have directly revoked anything and will likely result in legal battles, the answer to "how many times has this happened" is still "zero." As to whether Trump's actions are "unusual" - when aren't they?