Direct Answer: I could not find specific verbiage in ACOLO suggesting limitations on house representation on the Grand Council.
FWIW... if I'm following this correctly:
The High Council (per page 25, Absalom, City of Lost Omens (ACOLO))
Primarch Lord Gyr of Gixx (missing)
Acting Primarch Captain Wynsal Starborn
Captain of the Starwatch Asilia of Gyr
Chancellor of the Exchequer Scion Lord Kerkis of House Damaq
Chief Sanitation Commissioner Larrett
Commander Militant Chun Hye-Seung
Diplomatic Minister Ferridan Severus
First Lady of Laws Scion Lady Neferpatra of House Ahnkamen
First Lord of Spells Lord Gyr of Gixx (missing)
Acting First Lord of Spells Garethal Brighteyes
Harbormaster Scion Lady Adrielle Nephrathep of House Fyrlenn
Kortos Viceroy Jaress Molinarro
Sea Lord Lerefys of House Kethlin
Trade Minister Grenduul Fleng
- I only see one House of Damaq seat (Scion Lord Kerkis)
Lady Nymara and Seleenae each have seats on the Low Council
Lady Nymara representing a Coins Seat (pg 28, ACOLO)
Lady Seleenae representing a Western Council Seat as Nomarch (pg 28, 213, 220 ACOLO)
- Both being of House Damaq
The quote:
Twisting a stone on his ring, he drops the zone of truth, and then sits down himself. **"As you may or may not know, many of the noble houses in the city have loyalties that extend to the outside world. It's generally an open secret, everybody knows about it but nobody talks about it. None of the houses have more than one Council seat, and even put together they are outnumbered by the rest of the Council, so the balance is maintained."**
- Might be a reference to the Grand Council and the Low Council
(Thank you. Fun little PDF lore research project)