Technically and since Oracle RAC 10g Release 2, 100 nodes are supported in one cluster. This includes running 100 database instances belonging to the same (production) database on this cluster, using the Oracle Database Enterprise Edition (EE) with the Oracle RAC option and Oracle Clusterware only (no third party / vendor cluster solution underneath).

Note that using the Oracle Database Standard Edition (SE), which includes the Oracle RAC functionality, further restrictions regarding the number of nodes per cluster apply. Also note that one cannot use a third party or vendor cluster for an Oracle Database Standard Edition based Oracle RAC cluster. For more information see the licensing information.

When using a third party / vendor cluster software the following limits apply (subject to change without notice): 
Solaris Cluster: 8 
HP-UX Service Guard: 16 
HP Tru64: 8 
IBM AIX: 
* 8 nodes for Physical Shared (CLVM) SSA disk
* 16 nodes for Physical Shared (CLVM) non-SSA disk 
* 128 nodes for Virtual Shared Disk (VSD) 
* 128 nodes for GPFS 
* Subject to storage subsystem limitations 
Veritas: 8-16 nodes (check w/ Veritas)

Node limitations should always be checked with the cluster software vendor.