20,860

The Gregorian calendar is set up so that a year is equal to one revolution of the earth around the sun.  The months don't correspond to anything.

A month in the Muslim calendar is equal to one cycle of the moon, between 29 and 30 days.  A year is 12 months.  This means that a year in the Muslim calendar is about 11 days shorter than a Gregorian year.  This is why the Muslim holidays are not connected to any seasons.

(The Jewish calendar has the same months as the Muslim calendar, but some years have 12 months and some have 13 months.  So the year is roughly the same as a year in the Gregorian calendar.)

So at some point, the year number of the Muslim year will catch up to the year number of the Gregorian year.  This will happen in the year 20860.