DH totally Non-Cannon explanation to the the number of Galaxy class starships.
The original run was a 6+6 order. It was a experimental design, so they didn't want to have too many lemons. The design after teething problems with the warp core (Look how many Chief engineers the Enterprise ran though the first year.) and other issues, the +6 was ordered, along with say, 3-4 "Keels" being laid down a year. Some other related designs (Nebula) that used a lot of the same technology was also put into series production.
The mass of starfleet at this time (Season 3 TNG) is still Miranda's, Excelsior, and ambassador classes, with a slew of older types. The Fed-cardassian war, Romulans being active, and the Borg threat all showed some serious issues with the lineup of the Federation fleet. the older Oberth and Constellation classes where out of date, and the Miranda's and Excelsiors needed major refits.
There was a major attempt to upgrade the "backbone" of the fleet, the Excelsior, with major upgrades, as seen in the USS Lakota, and some Excelisor class-Lakota refits where still being built though the dominion war (as it was a wartime emergency) but most of the production went into the Nova/Nebula/Galaxy style of starships that where proven and mass production (which started after Worf-359, who numbers came to good effect in the Dominion conflict.) I would put the Intrepid class in the same era as the Nova/Nebula/Galaxy classes, with perhaps some upgrades.
Starfleet wasn't going to get caught behind the times again, and newer classes like the Sovereign being produced and being fitting out for yet another group of series production. Due to the losses of the Klingon, borg, and Dominon, the current production ships like the Nova, Nebula, and Galaxy classes are being produced in much higher numbers then even the most high expectaions. By the end of Voyager you still may have upgraded Lakota-style Excelsior's being built as a stop gap measure, as there are 100's of Miranda and Excelsior hulls and crews lost in the last 10 years and the Excelsior due to its long life is a inexpensive design that when upgraded can hold its own vs. Defiant class ships, while being much more suited for scientific study.