First off, welcome to the Forum K4NorthHaven.
I have the compact Weber chimney and it's the perfect size for use with an 18.5" kettle. Per my 1980's Weber owner manual, the 18.5" kettle requires 16 standard briquettes for each side for indirect cooking. Lighting the charcoal using a compact chimney, 32 to 35 briquettes fit in the compact chimney. Any more than that will spill over the top.