How does Ruby ensure only one instance of each number?

a = 1
b = 1

Both a and b refer to the same object.

p a.object_id 
 => 3

p b.object_id
 => 3

So when you're comparing a and b Ruby compares the object ids (as it does with symbols) which makes comparison performant.

The object_id's of integers, interestingly enough, are always odd. Everything else has even object ids.

Note the comment from Holger Just below which goes into some detail regarding large integers.