It is actually stated in two sentences as shown below:
Originally Posted by Gerald C Koch
Do not exceed the capacity of the fixer
Residual silver depends on fixer usage. There are three ways of keeping the residual silver to acceptable levels.
1 Fix only a few prints before replacing the fixing bath (approximately 10 20.3x25.4cm (8x10inches) prints in one litre/US quart of working strength fixer).
2 Use two-bath fixation.
3 Use a single fixing bath plus a washing aid. The number of prints through the single fixing bath can be increased to approximately 40 20.3x25.4cm (8x10 inch) prints per litre/US quart of working strength fixer.