That's not really true. NX-02 was already in development when NX-01 launched.
I'll repost something I post elsewhere.
Take it with a grain of truth, its a lot of words for what I now realize doesn't say much XD XD.
So NX Production - decided to peruse the wiki.
1. Enterprise - Unknown start. Likely post-2145 since the NX prototypes didn't break Warp 3 till then. Launched April 12, 2151. Launched 3 weeks early.
2. Columbia. Unknown start, half-way built by by March 2153. Completed June 2154. Delayed until November 2154.
3. Challenger. Unknown start, half-way built by May 22, 2155. Completed August 2155.
4. Discovery. Unknown start date. Completed August 2155.
5. Atlantis. Unknown start, half-way built by by 2155. Completed by late February 2156.
6. Endeavor. Unknowns start. Completed by June 2, 2156.
7. Intrepid. Unknown start. Completed by 2159.
The NX-01 can be said to be the testbed for the future NX's. Its successes led to the start of the NX-02, 03, and 04 by 2153. The Columbia mostly likely started in 2152.
Atlantis and Endeavour were probably started post-Xindi Crisis 2153-4 and Intrepid was the last NX to be built while production was shifting to Daedalus'.
But! NX successor Columbia-class had a faster build time with 4 of them being built within a year's time. Noice.
The loveable Intrepid was apparently a backup incase the NX failed, but went on to a long and illustrious career. The Intrepid itself was completed by 2153 so it probably started same time as the NX or after its completion since it reused many of the same components.
By April 24, 2153, at least 1 Intrepid and 2 Ganges/Warp Delta's were completed and combat ready. At least 2 other warp ships - reused CGI Steamrnners - also existed. Can probably
By 2154, with Enterprise returning, after ending the Xindi Crisis, defeating the Sphere Builders, and closing the 22nd century Temporal Cold War Front, at least 3-6 Intrepid's and 3 Ganges' were ready to defend Earth.