How do you track how many sessions a client has left in a package?

I sell sessions in packs — a client buys 10, then books them one at a time over a few months. Last week I genuinely lost track of whether one client had 2 or 3 sessions left in her pack, and ended up just giving her the extra one rather than get into it.

I found the "Offering a Series or Package of Events" thread (now closed for replies) — the redirect/daisy-chain workaround is great for pushing someone straight to their next booking, but it doesn't seem to track how many they've got left if they're not booking every session back-to-back.

For anyone selling packs on Calendly: how are you actually keeping count day to day? A separate spreadsheet next to Calendly? A different tool altogether? Curious what's actually working versus what people gave up on.