Will a lot of mods crash your game?

Depends on the game, the mods, and your computer. Mods that change the game's base files (damage done by certain abilities, changing the effects of certain items, that kind of stuff) shouldn't require more resources than the shipped game. Mods that add things to the game, be they cosmetic, items, areas, or quests/missions are draining. Some games also have more trouble loading mods than others.

Of course the greatest limiting factor is your hardware. If your computer has zero problems with the game as-is it can probably handle any mod you throw at it. If it struggles, I'd recommend against using resource-intense mods.

Yes, depending on the mod, the amount of memory it takes up, etc. But simply put, YES

Simply, yes.

Yes , and it depends on your cpu speed