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OK. Here's a bit of info on the guild structure.
Not to add to the confusion, but this is an error in the Book of Ti'ana.
The sentence on page 157 should read:
"The six assistant _Guild_ Masters, and the Grand Master..." 
There were many Guild Masters in each of the Guilds, but only one Grand Master for each Guild. Only a handful of the Guild Masters in each Guild were elected to the Council. The others had supervisory roles in their Guild or could be assistants to the Grand Master of their Guild.
The Great Lords were Grand Masters long before they were promoted. The Great Lords were still often referred to as Grand Masters of their Guild, though, this isn't their accurate title. Their title was simply Great Lord, and a new Grand Master was appointed to take their place in their Guild.
When a Great Lord died, a replacement could come from any of the eighteen major Guilds. The list given here (Writers, Miners, Books, Ink-Makers, and Maintainers - pg. 55) is a list of the five Guilds represented at this point in D'ni history.
How the replacement was chosen, whether it was seniority, age, popularity, some kind of vote, or another method, is not clear from what we currently know of D'ni history. My opinion is that it was probably a vote among the members of the High Council, but I could be wrong.
I feel that there were 18 GMs on the High Council, plus the Five Great Lords because of the way they are referred to sometimes in BoT.
You are correct again. The 18 Grand Masters are separate from the 5 Great Lords. So if you still count the Great Lords as Grand Masters (I do not, though David Windgrove often did), then you would say there are 23 Grand Masters.
Hope that helps.
- The phrase in The Book of Ti'ana says, "The six assistant Grand Masters and the Grand Master." This had led fans to believe that there was a Guild designation of Assistant Grand Master. RAWA corrected that misimpression as follows.