It varies, depending on the type of data and the availability of system resources.
TXNGroupmax
Specifies the number of files that are transferred as a group
between a client and a server between transaction commit points.
Client performance may be improved by using a larger value for
this option.
Specifying 0 indicates the node will use the server global value
that is set in the server options file. To use a value other than
the server global value, specify a value of 4 through 65,000 for
this parameter. The node value takes precedence over the server
value.
Attention: Increasing the TXNGROUPMAX value will result in
increased recovery log utilization. Higher recovery log
utilization may increase the risk of running out of log space.
Evaluate each node's performance before changing this parameter.
For more information on managing the recovery log, see the
Administrator's Guide.
If you're not having to do a lot of fail/reconnect, you're generally better off with a large number. The only way to know is to try backups at different settings.