If for brief periods in the day large numbers of people visit the forum and most of the rest of the day few visit one will still need a hosting program that can handle the large numbers of people that can come on briefly as if things are like this all the time. There is no built in surge protection in hosting accounts as far as CPU requirements. If you need X amount of resources to handle period Y then you will be paying for more resources throughout the month. If a webhosting plan does not fully cover "surge periods" of activity for an account then ones site visitors's will be mad and in webhosting having an active usuable website only 90% of the time should not be considered acceptable for anyone serious about their sites.

Search functions of forum software can require lots of CPU power. A few users searching can hamper the ability of others to just browse or post. Limiting search capabilities or watching them closely is a wise idea.

Message boards are hard to monetize. Ad revenue alone may not cover the cost of a high end server necessary for an active message forum.

The more users a forum has and the more posts it has the larger the database that will have to be searched or maintained so in general more users equals a need for a larger hosting account with more disk space and more CPU power.

Forum users can post anything a forum operator can only moderate things after they are posted. This opens up potential liability should a company not like what people on your forum are posting.

Message Board Hosting Options

If you plan on using a given forum software its a good idea to see how your webhost stands on this sort of issue. If your forum is going to be very busy you might require a dedicated server. But if you plan to try to use forum software for a small forum with virtual hosting one should check to see that the given server requirements for the software are available by your host.