Okay guys if we're gonna really try and figure this out, we need to be all mathematical about this shit! Hurling insults will get us nowhere!
Lakai started in 1999. In order to sell 1,000,000 pairs of shoes they would have had to average 52,631.5789 pairs of shoes per year. (Technically more than that because they weren't operating for the entire calendar year of 1999, but since we don't know what month they hit skate shops let's just count 1999 as an entire year.) 52,631.5789 pairs of shoes on average globally does seem realistic, yes?
However, the original statement in contention is that Marc "helped sell millions of pairs of shoes". So we can't take into account the years that Marc didn't ride for Lakai. Marc joined Lakai when? 2003? So assuming that his very presence on the team in some way helped sell every single shoe Lakai sold from that point forward, 1,000,000 shoes over the course of 13 years when MJ left for Adidas in 2016 would average to 79,923.0769 pairs of shoes sold per year. That number seems a little high to me.
In addition these numbers are based on broad generalizations as to how you define "helped sell".
In summation:
Has Lakai sold one million pairs of shoes? Maybe.
Has Marc Johnson helped Lakai sell one million pairs of shoes? Most probably not.