The Organizations in CoE are described as:
SCHOOLS: These are organizations dedicated to the furthering of education, research, and technology. Their org structure consists of some members of the organization who pay to be members (to receive training and to gain access to the school's resources) while other members of the organization are paid to be a part of it (as trainers and researchers). Their funding comes from a combination of their paying members and grants from the Kingdom, Duchy, County, or Settlement they reside in.
The main advantage to being a part of a school is that they offer increased and rapid opportunity for learning. This comes in the form of consolidated access to skill trainers, and also access to specialized research tools. And, as new technology is discovered, all students immediately gain access to the new knowledge. Because their primary benefits come from having a larger number of members in a single location, few schools want to create multiple locations, preferring to increase in size and scope at a single site or campus.
Because knowledge is power, and people are encouraged to focus their skill development on one set of skills at a time, characters (ie. identities) are restricted to enrollment in one school at a time.
GUILDS: Despite the same word being used, Guilds work differently in Chronicles of Elyria than in most MMOs and instead function more like guilds on Earth. Where in most MMOs they tend to recruit members with a variety of classes and crafting abilities with which to outfit the group for raids, Guilds in CoE tend to be about crafting goods or services, specializing in one or more trade skills, crafts, or professions - such as Smithing Guilds. Their org structure works similar to businesses on Earth, with employees and owners.
Revenue for an organization comes entirely from its members, with most organizations allowing its employees to keep a percentage of profits and depositing the rest into a central lockbox. Organizations rarely receive money from government, and, more often than not, must pay income taxes. Unlike schools, there is only so much business to be done in a single location and the most successful organizations will find increased wealth from setting up franchises or satellite locations all across their kingdom.
The main advantage to being in a guild is increased access to resources, and a steady stream of customers who trust the brand name established by your guild. As your guild's reputation and fame increases, so, too, will your revenue. As with schools, knowledge is power. Because most guilds don't want their trade secrets shared, players are restricted to one organization per type at a time. This means you could be part of both a Tanning Guild and Smithing Guild, or both a Smithing Guild and a School.
ASSOCIATIONS: These are organizations which generally provide a service to the community or to the governments. These range from preservationists to religious organizations. Members of an association come with varying skills and capabilities as, more often than not, the services they provide are unrelated to the hard skills of their members.
Similar to guilds, funding for an association typically comes from its members. Unlike guilds, however, all revenue from the members are passed up to the leadership to be redistributed or consumed as necessary to run the association. Because it is expected that all revenue is consumed for the purpose set forth by the association, these organizations typically do not pay income tax. That said, they're closely monitored by governments and settlements and, if they're perceived to be running a guild instead of an association, you can rest assured people will come to claim the money they're owed.
There are no common criteria around size and distribution of associations, as their purpose dictates such things. There are also no restrictions on the number of associations a character may be a part of at any given time.