I know there is a way to make grenades throw projectiles when they explode, but how is this achieved? I'm working on a spike grenade in case your wondering.

In the explosion tag, make it spawn a .projectile at different angle.

explosion damage effect or just explosion effect?

damage effects control damage. Effect tags are what control the effect of the given object

So where in the tag would I add an effect like that?

Open the explosion.effect for your spike grenade. If you don't have one, just copy the frag grenade effect over.

Go to PARTS and click Add.

Make it appear in any environment and either mode.

Make the location 0. effect_locations_block.

In type, change to projectile. Next to it, link to your .projectile for the spike.

The stuff below edits the speed and angle I think. Never messed with it much.

How do I determine how many projectiles its going to throw?

By adding the number of projectiles.
Make the first part of the effect, then duplicate it however many projectiles you want then just change the angles so they shoot off in different directions.

Do radius modifier bounds have anything to do with this?

Change the cone angle

I've been messing with the tag a bit now. How far does the angle go? Should I put them at very different angle (like 37-45-81-26), or like 1-2-3 sort of angles?

it goes to 360, i believe