Unfortunately, the number of people that will attend your wedding is very limited, considering all of the people you encounter across Skyrim. It seems that you can have up to 7 guests (rare, from what I've seen), but 5-6 is a pretty good number. If what others have said is true, you can have 0 guests.
Users in this thread reported between 3 and 5 guests.
Users in this thread reported up to 6 guests.
Users in this thread reported up to 6, but as low as 0 guests (not counting the priests that live there).
A user in this thread reported 7 attendees, but everyone else reported less.
From personal experience across a handful of save files, I've only ever seen 4-6 guests, picked seemingly randomly (Ysolda came to my wedding with Sylgja, for example).