Having multiple employers for your O-Visa petition is by no means necessary, but can be beneficial.
Employer letters are a significant part of the petition, because they sponsor you and emphasize you are extraordinary ability and how that would be beneficial to the employer in the United States. These well-drafted letters can help establish your recognition in the field, so the more letters there are that all speak to your talent can solidify that.
That said, it is not helpful to have numerous employer letters that are not very substantial. It is more important to have two to three distinguished employers than to have as many employers as possible to fill up your itinerary.
If you do decide to have multiple employers for your O-Visa petition, make sure that the employers are within the same industry. It is imperative that the job positions for multiple employers must be the same, or very similar so that you are qualified as having extraordinary ability in the same field.