Note that I have only just glanced over the Kingdom Builder rules, but I see two aspects to this question.
On the one hand, as was stated by SevenSidedDie earlier, which I will just full quote here for simplicity and completeness:
Farms aren't individual farms in that passage, they're a type of change to the hex. You can only add farms to a hex once—after that, the hex has farms.
One the other hand, there's this bit of text in the Watchtower entry:
A Watchtower flies your flag, is a safe place for your patrols, and establishes your power on the frontier. a Watchtower cannot share a hex with a Fort or another Watchtower.
The fact that it is explicitly forbidden to "stack" watchtowers on a single hex, implies that it is not forbidden to have multiple farm improvements (or roads, highways, fisheries, etc.) on the same Hex.
So as far as I am concerned:
By RAW, you can. The Rules don't mention a limit, even though they explicitly do in a (mostly*) equivalent case.
By RAI, you cannot. The rules not mentioning any kind of limit leaves us with exactly two non-arbitrary** choices: "One", or "None"
However, common sense dictates that there is limit on the number of farms on a single hex, because there is a natural limit to the area, and a less natural limit to the efficiency of your farms (due to technology, magic, amount of slaves you can efficiently throw at it, etc.). Similar logic can be applied to the other starred improvements as well, e.g. once you have turned the whole hex into a parking lot, you can't place any more highways.
*Unlike Farms, Watchtowers grant a one-time bonus as well.
**For any other limit N, you could equally justify N+1.