Multiply the width of your slide mount x the number of slide mounts plus 1/4" for each gap. You will have one less gap than your mounts—7 mounts = 6 gaps, 5 mounts = four gaps etc.
So for example; 7 mounts of 2” wide each would be 2” x 7 slides = 14”+ ¼" gap x 6 times = another 1½" would make it 15 1/2" in total for 7 slides - add another couple of inches to allow for it becoming shorter as its folded - any excess can be trimmed off the ends when you are done
Taking the credit card paper you made—cut one piece the length you have determined and triple the height of your slide mount. Fold this in half lengthwise (hot dog fold) and with the glue stick glue it together so the front and back are your credit card paper, raw white to the inside. If you used A3 one piece will be long enough but if you used A4 or ‘letter' size paper you need to join two pieces together overlapping them slightly - when joining you need to make it about 2” longer again so you can position the join under a slide mount.
When your paper is glued together trim the strip back at the cut edge so its the height of your slide mount plus 1/4" extra top and bottom (2 and half inches for a 2" slide mount) this will give you an nice edge at the bottom