We have another update on the total lifetime sales of the Metal Gear series. As of the end of March 2018, over 53.8 million copies have been sold worldwide.
A year earlier, by the end of March 2017, the series lifetime sales was at 51.3 million, so that means another 2.5 million copies have been sold in the previous year. One new Metal Gear game was released in that timeframe; Metal Gear Survive (February 20th / 22nd), but from what we can tell it did not do that great in the sales charts, so how many of those 2.5 million can be attributed to Survive is hard to say.
In any case, it seems like the franchise continues to sell admirably.
Source: Metal Gear in Concert Promo Video