Could they add more? Possibly, but I would not go into it counting on that happening. They may add additional times for the same excursion if demand warrants it.
Pdf Version - If you did not get one with original invoice ( which most likely you didn’t as assume you booked more than one year in advance ) I’d call your TA to have them check with Oceania to have a pdf version emailed. If no TA, just call Oceania directly to have it emailed
The $1000 shore excursion credit is actually broken down $500 per guest, especially if under SM. If you look at your original invoice it should show as such. So that would be minimum of 6 excursions per guest to qualify for the 25% discount. Once you have hit the 6, any above and beyond 6 would also qualify for the discount.
If I’m incorrect, I’m sure someone will be more than happy to let me know. 😉