When you reserve some but not all slots on an event for a particular user group, generally the remaining slots are left open to Unaffiliated Users, meaning anyone that's not being tracked as a member of a special group for that event. When the remaining slots are left open to Unaffiliated Users, that means they aren't reserved for a particular group of volunteers and can be taken by any user. In other words, members of the group you've reserved slots for will see the total number of spots they are eligible to register for, including Unaffiliated Users slots. 

To make it so that only individuals can see/register for the unreserved slots on an event, you'll need to reserve the remaining slots for a specific group of volunteers (e.g., "Individuals"). If you do not yet have an "Individuals" user group, create it first. Then, in your Default landing page, choose that user group in the Workflow section, under the "WHEN ANY USER VISITS THIS LANDING PAGE, ADD THEM TO THE FOLLOWING USER GROUPS" setting.  This will add users who go to your Default landing page (and therefore are, in most cases, individuals not using a custom landing page) to the "Individuals" user group, which will enable them to sign up for the remaining slots.