Number of Community Toilets Calculation

I design and build sustainable, environmentally friendly projects that bring clean water, proper sanitation, and good hygiene to rural communities.  I'm trying to determine how many toilets are needed for a rural community. Is there a formula to calculate this? Which factors weigh the most in the formula, in addition to the number of people?  Is the number different if toilets are at the household level and community/group toilets?