868681 hexes.
Think about it like this: there is one hex in the middle. Around this there is a ring of 6 hexes, the next ring has 12 hexes, the next has 18 hexes ... and so on. The number of rings is called <HexSectorSize> in the MapSizeDefs.xml 380 for insane. Thus you have:
1 + 6 times the sum of all n from 1 to 380
This sum can be expressed as (n+1)*n/2
https://en.wikipedia.org/wiki/1_%2B_2_%2B_3_%2B_4_%2B_%E2%8B%AF
Thus you have 1+6*(n+1)*n/2 with n being HexSectorSize. Have fun calculating out the numbers for the other sizes; free math lesson by zu, you are welcome 
Also: Insane has a maximum of 600 habitable planets. Ask them if they ever had a game with 600 cities 
