Spells should always be favored over minions when constructing arena decks. Especially if the are really good and cost effective damaging spells like swipe.
Swipe means 4 guaranteed damage to any one, for only 4 cost
+ you get to deal 1 damage to everyone else
+ it has great synergy with spell damage
How many are too much?
You cannot use swipe the first 3 turns and if you have alot of swipes and not enough draw cards, you will run out of cards fast. That is the most common reasons for losing in the arena.
For a balanced deck your arena should have about
5/30 draw cards at least
10/30 normal minions at least
5/30 other spells
So you can have about 10 swipes and still have a healthy balanced deck
Cost effective usually means several things: You spend less mana to take out a higher mana card and/or you are able to do a 1 card for 2(or more) swap. Swipe can accomplish both in the right situations And the fact that you can use it against heroes makes it a good finisher in a pinch. z-