hello,
how do i count the empty columns in a table and assign it to a variable.

I have a table in my db, that keeps the items a user makes in his selection/subscription for items available.

PROBLEM;
1. there are a maximum of 6 options a user can subscribe to.
2. Minimum of 1

Once a subscription is made, the options are kept in a table e.g.

item1
item2
item3
item4
item5
item6

SOLUTION
Now, if a user subscribes to a maximum of e.g. 3 or 4.
I would want to be able to count, the number of empty columns and the number of subscriptions and assign it to a variable.

How do i do this EFFICIENTLY

thanks
Afrika