The last time we looked at this, in 2011, only 10.8% of these domains were registered.
In other words, out of 11,881,376 domains, around 1,283,189 were already taken.
It's now 2018, and we re-checked a random sample of these (10 samples of 500 random character strings, so 5,000 total). It came out that an average of 29% of these domains were registered.
This means that registration of LLLLL.com domains has about tripled since 2011. There are now 3,445,599 5 Letter .com domains registered. Still less than half of the overall pool, but a steady stream of registrations nonetheless.
What does the registration look like for 7N.com's? This is somewhat of a parallel, because there are 10 million of these. A whopping 3% of these are registered, or around 300,000... Shows the dangers in trying to do a parallel between numbers and letters based solely on scarcity. Letters are still far more popular on a 1:1 basis.