Databases in R5 format (On Disk Structure, or ODS) can have up to 64,000 fields by enabling the option "Allow more fields in database" on the Advanced tab of the Database Properties box. Databases in R4 format and R5 databases that do not have this option enabled can have up to 3000 fields (where the total length of the concatenated field names is less than 64KB).
Enabling "Allow more fields in database" lets you create more fields in a database but keeps some features, such as full-text indexing, searching by form, and populating the field list in the Design pane of Domino Designer, from working. Also, replication between a database with the "Allow more fields in database" enabled and a replica of that database on an R4 server fails. You cannot create a new replica of a database that has this option enabled on an R4 server.
Lotus recommends that customers not enable the option "Allow more fields in database" for databases with replicas on both R5 and R4 servers.