I've read somewhere that each new city adds 30% to the cost of social policies. If we assume that new cities can produce roughly the same amount of culture as existing cities, then the following shows the relative cost of acquiring social polices based on number of cities.
Cities ----- Cost of social policies/number of culture produced
1 --------- 100%
2 --------- 65%
3 --------- 56%
4 --------- 55%
5 --------- 57%
6 --------- 62%
7 --------- 69%
8 --------- 78%
9 --------- 91%
10 -------- 106%
As can be seen from the above table, 4 is the optimial amount of cities to posess in terms of social policy cost per culture generated. Clearly there are other advantages to having more cities (extra science, gold, resources, etc.) so a player going for a culture victory will need to weigh those against the increased amount of culture required.
Cities ----- Cost of social policies/number of culture produced
1 --------- 100%
2 --------- 65%
3 --------- 56%
4 --------- 55%
5 --------- 57%
6 --------- 62%
7 --------- 69%
8 --------- 78%
9 --------- 91%
10 -------- 106%
As can be seen from the above table, 4 is the optimial amount of cities to posess in terms of social policy cost per culture generated. Clearly there are other advantages to having more cities (extra science, gold, resources, etc.) so a player going for a culture victory will need to weigh those against the increased amount of culture required.