Trump made the claim immediately after his inauguration. All evidence pointed to the contrary. Sean Spicer defended the claim the day after, as well as other unfounded claims about voter fraud.

Spicer was replaced by Anthony Scaramucci six months later.

The Atlantic – Trump’s Press Secretary Falsely Claims: ‘Largest Audience Ever to Witness an Inauguration, Period’
Politico – Trump fumes over inaugural crowd size
Vox – Photos: the crowd at Donald Trump’s inauguration vs. Barack Obama’s
The Hill – Trump admitted he was wrong over inauguration crowd size fight: book