This is an awful lot like the "How many piano tuners in Boston?" question.
So I'll make up some numbers. Let's say that 1 in 20 US teenagers want to make music. 350 million teenagers and former teenagers (including the deceased born after 1940), 17 million musicians.
Let's say that 1 in 20 musicians want to play the bass at some point. 900,000 wannabe bass players.
Let's say that 1 in 20 bass players stick with it. 50,000 actual bass players.
Let's say that actual bass players collect an average of 5 basses. 250,000 basses.
So, in the US alone, that's 850,000 (wannabes) + 250,000 (collectors) = 1,100,000 basses in the US.
The US is about 10% of the world, so 10 million basses in the world.
Every one of those figures is wrong, but I'll stand by the final result. There are 10 million basses in the world, ±100%.