you may get flamed for the question, but yes, people alter the # to get reservations. Sometimes, that is the only way to get a table, especially since the way the Disney reservation system works it seems it's very particular about exact numbers at tables. There may be 20 tables for 3 available but it will not put a party of 2 at one of those tables, no matter what. Even if the tables for 3 sit empty on the system.
And actually, back when all reservations had to be made on the phone with Disney Dining, there were often reports on this forum that CMs were suggesting that an ADR "might work if you had 3 instead of 2...or 5 instead of 4" They didn't outright suggest you lie...just that they would make suggestions that you "might want to find another person to go with you because the bigger table was available...and then it would sure be "too bad" when that extra person couldn't make it to the dinner with you." The CMs knew the system was too rigid.