The Spell Glyph section of Glyph of Warding states:
The spell must target a single creature or an area.
Some spells have the potential to affect "up to X" creatures. The Slow spell, for example, states:
You alter time around up to six creatures of your choice in a 40-foot cube within range. Each target must succeed on a Wisdom saving throw or be affected by this spell for the duration.
Can you make a Spell Glyph of Slow (or any other "up to X targets" spell), if the intent is to only target the creature who triggers the glyph?