Wow impressive one there BKB! Am I assuming that it was one of the coins from Dmitri Markov auctions?
Regardless, if I am not mistaken, the source that I quoted it from was from Konros, "History of Monetary striking in Russia" 2nd Edition, St. Petersburg, 2000.
The source appearently goes on to describe that in 1839, 160,000 1 ruble coins were struck, and 6000 of 1.5 rubles. But in 1840, because of a serious lack of demand, 10,000 of such coins are to be in Moscow and another 10,000 to be in St. Petersburg. These are supposely only for military commemorative coins, but appearently the solders were already paid in common rubles. There are no mention of 1.5 rubles to be kept and I don't know why. And indeed, there was an order to destroy the rest of the 140,000 and whatever 1.5 rubles there are.
But remember, the denomination of 1.5 rubles is a complete foreign idea to most Russians. The denomination was only created in 1833, and that was actually a dual denomination with the Polish coinages, and is always denoted as 1.5 rubles - 10 zlots. And in 1835, the first commemorative ruble in such denomination was struck. I know how impressive that coin could be - afterall it's a massive one ounce coin!

As of how many of 1.5 rubles exist, I don't think anyone would really know as there is no futher details if such culling did occur, and if it did, there are often no details of how much were melted.
There is some other interesting thing about 1.5 rubles struck in St. Petersburg. It seems that after 1838, such coins are actually difficult to find for some reasons. Is this because St. Petersburg mint decided to strike a lot less of these coins? But are these related to the striking of the Borodino rubles and cullage orders? Not only that, normal coins from 1838 to 1841 all faced some kind of issues, and 1841 was the very last year, that the huge denomination 1841 was to end. Did St. Petersburg mint actually struggle to meet with demand and actually had supply issue? Was the culling of Borodino rubles necessary to keep up with the demand?
Guess there are plenty of questions there