That may be unknowable with any degree of certainty, but we can make an educated guess:

Say there's 4GH/s total and the top 26 miners account for 20% of that, as per the pie chart that I so love posting. The smallest of those 26 miners has 0.2% of the total (I don't know what rounding might have went on but we'll go with it) which is 8MH/s, or about 6 750Ti cards or equivalent.

The other 80% of the total, 3.2GH/s, could be made up of

400 miners with 8MH/s each
800 miners with 4MH/s each
1600 miners with 2MH/s each
3200 miners with 1MH/s each

... but is almost certainly a distribution curve with larger numbers at the bottom, having, individually, smaller hashrates. Remember a lot of people still CPU mine just on the offchance, like you with your Antminers on BTC.

So I don't think I was way off with my 'thousands of smaller miners' comment. And any way you slice it it's at least hundreds of times more decentralised than just about every other PoW coin.