You can never have too many heralds to share the duties. The main thing to remember is that some people are book heralds, some people are vocal heralds and some people do both. Don't try to fit someone into an area they are not interested in. For those interested in vocal heraldry, there are more choices. When seeking help, don't be afraid to ask for help from those arriving at the event.
Many people come to events wanting to assist.
If you announce early that you are looking for people to help herald, you will get several volunteers. Be sure you get the names of everyone who helps, so you can thank them later. If at all possible, separate the area for the consulting table from Herald's Point. Herald's Point is where the populace goes to request announcements to be made, to arrange business in court and have field heralds conduct tournament cries. The Consulting Table is where the populace goes to receive assistance in creating and submitting persona names and devices.