I consider the best practice to be "you want the same LUN presented to every host in a cluster". There are limits to the number of LUNs you can present to each host, often 255, but also often SAN dependant. Once you get anywhere near this, you should be creating a new cluster for a second set of LUNs.
The other thing to consider is the current limit of 32 hosts in a cluster - obviously you can't exceed this, and larger numbers are better handled separately.
