How much you add depends on what you are going to be doing. If you are closing the pool, then you follow the procedure in this post that uses the maximum dosage on the label which is 16 ounces per 10,000 gallons. So 18,000 gallons would be 28.8 ounces so just under 1 quart (so yes, 1 quart should be OK).
A fellow by the name of Ben Powell, of poolsolutions.com--well known to all who have been on this site for any length of time, has this info on his site:
Buckman Laboratories, in Memphis, TN, makes all the polyquat sold in the US, regardless of the label. Arch(HTH), BioLab (Bioguard, Omni, Pool Time, Aqua Chem, Sun, many others), LaPorte (Robarb, etc.) ALL simply repackage -- and possibly, dilute -- what Buckman makes.