Double slits parallel to the rim, aka "snake bites," are usually the result of a pinch flat: e.g., underinflated tire meets pothole.

Single slits mostly result from using tire levers (or screwdrivers, etc.) to install the tire and tube. If at all possible, avoid using tools when remounting the tire and tube.

And make sure that no part of the tube is caught under the tire bead.

Inflate to 15 psi or so, check the tire bead again, confirm that the witness line above the tire bead is not too low or too high all the way around both sides of the tire, and then fully inflate.