There is no limit on the number of 'distinct' artifacts that can be used on you at one time, but each artifact is a little different. An enemy cannot use the same artifact on you again if you are already affected by the same one.
Yes, artifacts can easily turn the tide of battle in many areas - its worth investing in them, especially the more powerful artifacts. You should stockpile them so you can properly counter-attack when necessary. You've probably only seen the more common artifacts.
I'm sure you'll get your revenge
