The 84/ 85 figure for videos came about as the result of a FOIA request for videos showing the Pentagon impact. FBI Special Agent Jacqueline Maguire responded with the following point:
...I subsequently searched a series of FBI evidence databases, including the FBI's Electronic Case File system and the FBI's Investigative Case Management System, and determined that the FBI possessed eighty-five (85) videotapes that might be potentially responsive to plaintiff's FOIA request. This determination was based on videotapes that had been submitted into FBI evidence, sent directly to the FBI laboratory in Quantico, Virginia, and/or obtained by the FBI's Washington Field Office.
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The FBI are talking about 85 videos, but this is just the result of an initial search that includes (for example) all videos obtained by the Washington Field Office. If we move on from that then the numbers begin to fall dramatically.
56 "of these videotapes did not show either the Pentagon building, the Pentagon crash site, or the impact of Flight 77 into the Pentagon on September 11."
Of the remaining 29 videotapes, 16 "did not show the Pentagon crash site and did not show the impact of Flight 77 into the Pentagon."
Of the 13 remaining tapes, 12 "only showed the Pentagon after the impact of Flight 77."
Only one tape showed the Pentagon impact: the Pentagon's own security camera footage, that would later be released.