You gain a 'pool' of Glyphs for each (multi)class
As written in the Mutliclass Archetypes' Dedication entries (here, Wizard)
You’re trained in arcane spell attack rolls and spell DCs. Your key spellcasting ability for wizard archetype spells is Intelligence, and they are arcane wizard spells.
It seems to create a clear line between the spell slots (and this is supported, in a way, by the fact that the Spellcasting Archetypes indicate you cannot cross-use spells) and assigning your spellcasting ability for those spell slots.
By your examples
Wizard with Bard dedication and basic spellcasting. Int mod is +4, Cha mod is +3. Can he create 7 or 4 glyphs if he can cast it both as an arcane and an occult spell?
7 Glyphs; You can cast four glyphs as a Wizard* and three glyphs as a Bard*.
Cleric with Druid dedication and basic spellcasting. Wis mod is +4. Can he set 8 or 4 glyphs if he can cast it both as a divine and a primal spell?
8 Glyphs; You can only cast four glyphs as a Cleric* and four glyphs as a Druid*.
(*)When casting a Glyph using that 'pool', you use the appropriate class's spell slot (and by extension, available spell levels) and DC (for detection and disarming). Multiclass DC will tend to be lower due to proficiency, as well as potentially having a different ability score. You're also limited in the spell levels it can store.
This seems to be the most accurate answer I can find, applying the Rules as Written. Because the rules are somewhat vague, expect table variance on this issue unless it becomes FAQ'd.
