"Le" is a pronoun and it serves as a "complemento indirecto" it tells us who is receiving the action later in the sentence which are the clients, because you show the houses to the clients, so they are at the reveiving end of that action. Also that sentence would sound better if you change the order a little bit say that instead of saying "Normalmente le muestro a cada cliente veinte casas" you say "Normalmente le muestro veinte casas a cada cliente" that way you express the complete action, that you show twenty houses, and later to whom you're showing the houses which would be to the clients. That would justify the "a cada" because you would be saying Normally I show twenty houses to each client. The order which you say things changes a little when you translate from English to Spanish. Hope this helped!