The official VAT statistics (thanks Mark Purdue) show that there are 1.2m VAT registered businesses with turnover above the registration threshold. Looking at this in conjunction with other official stats, my calculated guess is that the 1.2m includes 1m limited companies.
In my experience, limited companies and VAT-registered businesses generally are much more likely to use accounting software or, at the very least, Excel. So the percentage of the 1.2m for whom MTD is going to require a change of record keeping system must be pretty low - 20% at most?
So, (stay with me here) if 20% of 1.2m = 240,000 and the total UK business population is 5.5m then MTD, as it is now proposed, will require maybe 4% of UK businesses to switch from non-digital records.