I also have the occasional client with ‘leftovers’. I usually contact the client and also have a ‘gifting policy’ as Paul suggests.
I used to sell packages of 10 sessions. I put a 3 month expiration on the packages. But I was not strict in enforcing it. And that made it worthless to do because if you let it slide it means nothing legally. So, I dropped selling packages. Now I charge clients monthly. Those scheduling more sessions per month get a discount. At the end of each month, I give each client their total charges. It makes everyone’s life easier in my opinion.