Greetings and welcome aboard, @TH_CH_TH_O_HOANG 
Glad to say that we’re currently working on the opportunity to add custom fields to the booking form, which should cover your use case. I’ve moved your comment to the related thread, where we’ll keep you updated 
Also, we have a request for the multiple appointments for the same time slot - Multiple appointments for the same time slot.
Many users mentioned the opportunity to specify the number of guests for the table reservation. So, feel free to upvote this idea as well - Multiple appointments for the same time slot