There are several ways to make plausible comparisons. The simplest is to compare the Brexit party’s 5.2m votes across the UK with the Lib Dems and the pro-remain Greens, who attracted 3.4m and 2m. Taken together, they come out 142,000 votes higher at 5.4m.
A second method is to take all the pro-Brexit parties – Farage’s party, Ukip and the DUP – and compare that against the pro-remain parties, taking in Change UK and Scottish, Welsh and Irish nationalists. That gives you 5.9 million voting unambiguously pro-Brexit and 6.8 million voting for remain parties, including 884,000 from the SNP, Plaid, Sinn Fein and the Alliance Party in Northern Ireland.