Enterprise NX-01 is Starfleet's first deep-space starship. For their to be a Starfleet, there has to be a fleet of ships! We've seen a few besides the NX-01. As of "The Expanse" (Season 2 finale), we've seen the Intrepid-type and the Warp-Delta. "Storm Front, Part II" introduced the Sarajevo-type as well. At the end of "Storm Front, Part II," we see a fleet of starships welcoming Enterprise back to Earth. There are Starfleet and Vulcan ships, and perhaps ships from Earth's allies. I count at least twenty ships.
In "Zero Hour," the only ships defending Earth from the Xindi Death Star where Degra's shuttle and Shran's battle cruiser Kumari. Where was Starfleet?