There is no clear cut way to say if you need a shared or dedicated exchange server, but here are some helpful points to consider.  Starting with a shared server is not a bad option. It is important to find out the ease of transition from shared to dedicated if you do find yourself needing more resources.  Below are the evaluations of a few main points for both shared and dedicated Exchange servers.

1. Number of users
2. Mailbox size flexibility
3. Bandwidth demands
4. Server uptimes
5. Contractual Agreements
6. Administrative control level

Shared Exchange Server:

Cost effective general Microsoft Exchange access.  Suitable for most small to medium businesses.

1. Suitable for small to medium businesses.  Server would also be shared with many other small to medium businesses.
2. Mailbox sizes are governed solely by the plan details chosen.  Flexibility is limited but may be more then suitable for your needs.
3. Based on the terms of your plan, bandwidth could be capped and may be set at a flat rate for overages.
4. Uptimes for shared servers are generally 99.5% to 99.9%.  Although reasonable, sharing resources with other companies makes you susceptible to outages.
5. Shared servers generally offer flexible contract free agreements.
6. Very little to no control is given to a shared server plan.  Administration is handled by the provider on their terms and conditions.

Dedicated Exchange Server:

Pricey highly configurable access to Microsoft Exchange for the advanced user or the larger company.

1. You control the number of mailboxes being used since you are on a server alone.
2. Allotment of mailbox sizes can be adjusted by you and the server will only contain your mailboxes.
3. Bandwidth allotment is generally greater but may also be capped.  An overage rate can apply for dedicated servers.
4. With the server being dedicated solely to your business, a 100% uptime is almost a certainty.
5. Since additional hardware is required for a dedicated server on the providers side, usually a 1 year contract is required.
6. Dedicated servers allow control over permissions, groups, rules, and access.  Some Exchange users do not need this extra administration, but in many cases it can be quite useful.