Read the articles on corruption but I can give some hints on what to look for...
First, understanding that corruption has 3 parts. The first is based on distance and the second is based on rank. Rank is how many cities are closer. The third is how many cities you have total. Communism only has corruption due to numbers of cities but the corruption rate is much higher.
Distance is 1 per tile straight out from your palace and 1.5 diagonal. So, 2 diagonal tiles is worth a distance of 3.
Next, if you had a city at a distance of 3,4,5,6 and 7 then the distance 7 city would have a rank of 5 because it is the 5th city from your palace.
The optimal number of cities a few more cities than when you are able to build a forbidden palace. That is, if you can build an FP at 10 cities then the optimal number of cities is around 12 or so. The optimal number is based on map size and is listed someplace on this board.