It really depends on which restaurants you are planning to eat at, when you are going, how many people you already have an ADR for, etc. If you are a big group and/or eating at the more popular places (e.g., character meals), then it might be difficult to get an ADR for a bigger group.
You should try to modify your ADRs because if you walk-up with an extra person, the restaurant might not be able to seat you.