How did they calculate such a high number? Were they running more than one monthly show at MSG during the 8-year reign? Two shows a month gets them there, but was he always the headliner if that was the case?

Most of the stories had the exaggerated statistic of him headlining 211 times in Madison Square Garden and selling out 187 times. Later, after the 2013 Hall of Fame ceremony, that number changed to being his 188th sellout.
The number isn’t close to accurate, but he did both headline and sell out Madison Square Garden more than any wrestler in history. He told me that he and Georgiann Makropolous came up with the number, estimating he regularly worked the building for 18 years, once a month, and sold out 90 percent of the time. This was long before research was done and real records were available.
His actual record of Madison Square Garden matches is listed here. He did 160 matches in Madison Square Garden, and was only pinned once, which ironically was his most famous match ever, and the most enduring match of that era, with Koloff.
Of those, 140 of those shows you could say he main evented. When he was champion, every match he was in was the main event. During the Morales and Backlund runs, his match had equal billing with the championship match as the main event. Often his match was a bigger drawing match than the championship match. For those who would watch the Madison Square Garden shows live, or on the MSG Network in the late 70s, some of the fondest memories would be right before the final match, when the ring announcer, where it was Johnny Addie or Howard Finkel, would announce the next show.
When Sammartino was champion, the anticipation was who would be the next contender, although usually that was obvious, or in the case of a disputed ending earlier in the night, what the special stipulations would be next month.
But after he lost the title in 1977, because he was not there every month, the announcement of his appearances would be louder and more dramatic.
Finkel was the master of this, and he’d run down the card, and then the championship match, and you thought it was over. He’d subtly stop, and then say that there’s one more match, and he’d announce that name of the heel, who was always one of the top guys. “And he will face,” as his voice boomed, “BRUNO SAMMARTINO!” The place never failed to explode.
Pay records from Madison Square Garden from the Backlund era showed that every show he appeared on, he was paid the most, always $6,000, which was more than Backlund, who would get $5,000, or Andre the Giant, who would get $3,000.
Legitimately, his match was the true main event a minimum of 130 times. Not all attendance figures are available, but he had 60 verifiable sellouts, and logically, there are probably a few others, but not more than 65.

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