What do they call a number that goes on forever?

I believe they call it a recurring number, for example, infinity is a recurring number.

You could say infinity, or infinite, but recurring is most used. for example, 3/10 converted to a percentage would be 33%, but as 33 × 3 doesn't go into 100 fully, we say 33.33% or 33.333%. the ".3"s could go on forever, which is why it's recurring.