According to the Star Trek: Star Charts (pg. 66), there were 23 Earth outposts. Outposts 1 to 8 were mentioned in dialog or were seen on the Z-6 Sector map in "Balance of Terror" and Outpost 23 was mentioned in dialog in "Future Imperfect". Outpost 3 was constructed in an area of space that been contested over during the Romulan War Theater of Operations (2156-2160).
This outpost's location near the Neutral Zone was seen on a star chart in 2366.