Here’s the answer via Dan Hodges:
Labour’s [up-to-date*] canvassing data covered five million voters
The UK electorate is around 45 million, which means on these figures the Labour Party has canvass data for a little over 1 in 10 voters.
Of course that’s an average for something which will be heavily concentrated in particular areas, but even so it’s a healthy reminder of the messy and incomplete grassroots reality behind the grand talk of big data and microtargeting which usually steals the spotlights and is talked of as if it is a mass communications tool reaching almost every voter.
As with Labour’s online advertising the reality is often far smaller and rather more humble: clever things reaching a small number of people that can only make the difference in a very close contest.
Success in making something scale to a level where it can have greater impact does happen sometimes, but it’s harder and rarer than the general messy reality.
* From the context, the five million refers to canvass data up-to-date enough to judge how someone voted at the general election.