I'm creating an encounter with a Glabrezu, which has Innate Spellcasting of Power Word Stun. What is the Save DC? It says that the Glabrezu's spellcasting ability its Intelligence; does that mean 8, plus 4 from Intelligence? Does it add a Proficiency bonus? If so, what is that proficiency bonus?
As mentioned by Erik, the Glabrezu's stat block does indeed list its Innate Spellcasting saving throw DC:
Innate Spellcasting. The Glabrezu's spellcasting ability is Intelligence (spell save DC 16). The Glabrezu can innately cast the following spells [...]
This probably solves your problem. However, this value isn't random - if it were not given, you could calculate it as follows.
Spell saving throw DCs are calculated as
DC = 8 + proficiency bonus + spellcasting ability modifier
The proficiency boni of monsters are listed in the DMG, page 274. According to that table, the Glabrezu, being CR9, has a proficiency bonus of +4.
The table also lists save DCs, though I believe those are more intended for saving against the monster's abilities, not their spells. Either way, for balancing purposes, one should probably adhere to these values when setting ability scores (and, subsequently, spell save DCs) for a custom monster. For example, the glabrezu's Innate Spellcasting has the same DC as listed in the table.
Hence, with an INT modifier of +4, the Glabrezu's Innate Spellcasting save DC equals
DC = 8 + 4 + 4 = 16
which is identical to the value given in the MM.