I have 2 SBs on my B list right now. For me, that means we don't have any regular schedule, but I know if I ask them for a date they'd be interested. One of them was previously on my A list. We had a very regular, affectionate thing going. When the pandemic hit she left town and quarantined with a gf out in the country somewhere, far from other people. She's back in town now, busy with her life, but we are still on very affectionate terms. We've gotten together twice in the past 10 weeks. It's great having her "on reserve" if my A list SBs cancel.
My other B list SB is a bit older than my usual. She is not in town often, but when she is, she often texts me asking if we can get together. Sometime I decline, due to a full schedule, and she doesn't seem to mind, just says great, seeya next time. So I don't think I've pissed her off by not being regularly available. We've had 3 dates so far this year. She does expect me to give her a bit more $ than my regulars, so if our arrangement should fade away I won't mind.
As for Lester's comment, yes, it does take some coin to maintain a steady rotation that you see regularly. I have 2 on my A list, each of who I visit every 2 weeks. But they have lives too! Right now, one is out of town for 3 weeks, and one had to cancel recently for other reasons. So although it sounds like I have a schedule of 6 dates per month with these 3, it often does not work out that way. And there's always the possibility that an arrangement will come to an end, for any number of reasons. Which is why I continue to go prospecting on SA. Plus cuz I enjoy it!