Answer:
there isn't a horizontal asymptote
Explanation:
A horizontal asymptote is a horizontal line (y-value) that the function approaches, but never reaches. The difference between the asymptote and the function value gets to be arbitrarily small.
This graph has a curve that never approaches the horizontal. It has no horizontal asymptote.
_____
Additional comment
This looks like it might be a graph of US population. If there is a horizontal asymptote, it would be at the carrying capacity of the land area designated as the United States. Estimates of that value vary considerably, ranging from 200 million to 807 million (at least). Climate change, pollution, disease, and available resources will modify these numbers and/or speed the approach to the actual value.