Depending on how your ISP is doing IPv6, the typical way would just route the entire prefix to your router, and the ISP would not know how many addresses are used, in the sense that there is no table. If your ISP is keeping data of flows from your prefix then it could start counting unique, but there isn’t really a reason to and it wouldn’t be accurate. Some devices pull and use more than one address, and quiet devices might not get picked up by flows which are typically sampled at a ratio.