There are 6 principal guilds in the game, most of which are subdivided into various specialisations. These may or may not have unique primary skillsets.
4 distinct specialisations.
7 specialisations (Priesthoods), but with common skills.
5 distinct specialisations.
10 distinct Specialisations.
A single guild.
8 different skillsets, spread across 19 orders.
New players start out as an adventurer, this is a "guild of convenience" to support certain game mechanics that require a player to have a "guild level" defined by the 15 "primary skills" assigned to a guild, however it is not a true guild in that it has no guild houses or advancement structure. While there is no requirement for players to progress from being an adventurer, it is MUD policy that no active development is directed towards those players who do not join a guild, and no consideration is given with regards to being disadvantaged by new game features and systems.
Notwithstanding MUD policy adventurers can be enthusiastic about their choice to choose no chosen profession. Like people who didn't get the invite to a really good party tend to be excited about whatever it was that they wanted to do anyway, adventurers understand that the people who have really missed out are the ones drinking champagne and playing twister. In any case, the choice of a guild is a life sentence, so it can be worth spending a little time as an adventurer, even if only to spend time learning which guild is the one for your character.