220

220 is the smallest amicable number.

The proper factors of 220 are 1, 2, 4, 5, 10, 11, 20, 22, 44, 55, 110.  They add up to 284.

The proper factors of 284 are 1, 2, 4, 71, 142.  They add up to 220.

This makes 220 and 284 an amicable pair.  Each one is an amicable number.

Other amicable pairs include 1184 and 1210, 2620 and 2924, 5020 and 5564, 6232 and 6368, 10744 and 10856, 12285 and 14595, 17296 and 18416, 63020 and 76084.

There are 4316 pairs less than 2.01x10^11.