How submarine numbers turn into patrol availability

Do SSBN fleets consciously operate on three- and four-unit rules?

Yes; four boats can sustain one patrol, and larger fleets can use the same cycle to keep several boats continuously available.

Four boats are a minimum structure that can guarantee one on patrol while the others transit, prepare, or refit. Larger fleets gain the benefit of numbers rather than simply repeating the same one-for-one rule. With twelve boats, he sketches a force able to keep about four at sea while the rest cycle through other duties.