From Monarchs Migrating through Ontario:

Almost 200,000 dead monarchs were found on 2 highways in Mexico in 2018

“Recent estimates based on a study in Texas suggest that mortality during autumn migration may be of the order of 2 million per year or about 3% of the population.”

Read the whole study:
Mortality of Monarch Butterflies (Danaus plexippus) at Two Highway Crossing “Hotspots” During Autumn Migration in Northeast Mexico