Hi Jon,
The number of domains really isn't an issue here unless the shared hosting provider charges per domain. The real issues that you need to take into consideration are 1) the amount of traffic your site will generate and 2) your own requirements for uptime if another site were to crash the shared server.
If you know your site is going to get a lot (I know that's relative) of traffic or you are not comfortable with the possibility of your site being down due to a crash caused by another site, you should go with dedicated hosting.
HTH,
Joe D.