Have a single decision maker as the main point of contact (both on your side and on the client side).
If you have multiple points of contact for your project, you, as the project manager, will not be able to control the flow of information properly.
If your client wants to make a change, clarify something, etc, they should have one specific person to communicate with in your organisation. If it is a small project where the project manager has control of all the areas, the contact should be the project manager. If a larger project with design managers, construction managers etc, you could have communication segmented, but each of these areas should only have one decision maker who responds to the client.
On the other side, make sure you have just one person who you contact for all enquiries, clarification etc at the client organisation (their project manager for the project). Communication could then be directed from them to others, but you need key decisions processed (and essentially made or authorized) through that one person.
Without this being done, you could experience decisions being reversed because the information didn’t come from the authorized person.
It is also a good idea to have these “authorized” people to be listed (by title at least, preferably also by name) in the contract. If a direction comes from the client from someone other than the authorized person, a copy of that direction (order) should be sent to the official person for them to authorize.
A single decision make and point of contact saves the backwards and forwards of communication.