To ascertain support levels, you look at the actual numbers of votes, which is also the end result and the more pertinent one, rather than at seat numbers. And the French National Front's vote numbers have declined significantly from 2012 and 2014.
UKIP got nearly 4 million votes in the 2015 UK general election and won one seat.
Despite them only getting one seat, it would have been foolish to say they did not attract a significant level of support.
Seat numbers hide the reality - you have to look deeper and look at the actual numbers of votes.
UKIP's collapse in 2017 to 1/7th of its 2015 level, incidentally, is the classic example of the collapse of the far-right vote across Europe.