Thank you for these details, Halia3236.
On your phone, are the messages appearing as normal group messages vs. individual messages? You're only seeing this on your online billing, correct? It can be that some phones only allow a maximum of 10-recipients. Therefore, if thee are individual messages showing up on your bill, it can be that there were more than 10-recipients in the group.
Does this happen with multiple group threads, or just the one?
Also, are you using the Messages+ app, or the standard Samsung Messaging app?