Yeah, it seems strange. It’s not me that translated it. It would be better to say:
多くの日本の寺院の境内で見かけることがあるだろう。
The English version shows it clearly, though.

The problem is that I think the translator didn’t consider it. See “in the courts of certain Japanese temples”, it’s plural, then should be 多くの日本の寺院において and next, “you may see…”, it means 見かけるかもしれない、見かけるだろう, i.e. probability, which can be said in other way, “sometimes you see…”, therefore there appears ときたま. It is actually paradoxical but this kind of mistakes often occurs in daily conversations, because, in this case, it is a fact that there are these mirrors in many temples, and consequently you have the opportunity to see them in practice, so first you may say よく but next ときどき. However ときたま is not needed here and, if the sentence was divided into two(寺の境内にはよくそうした鏡が置いてある, だからときたま見かける), there is no problem. Apparently it’s not intentional and understandable, no one cares.