You may use ecredits for a single reservation, however if the ecredits are 'attached' to your name (ie leftover value from a previous canceled ticket) then you can only use the amount for the respective person's ticket. If they are credits from volunteering on an oversold flight or from a cust service issue, then they are likely not tied to a single person and may be lumped together.

Normally there is a maximum of 3 credits that you can use for a ticket, so I'm not sure if that is still 3 max total for a multiple pax booking or of it allows you three per person.