FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
Article Id 192925
This article describes what is the maximum number of interface history widget supported in FortiOS.

FortiOS only support maximum of 25 interfaces history widget to be monitored.
If there are more than 25 widgets, the 26th Widget will not be able to load any data.

Verify if the maximum limit has been reached by running a debug:
#diag debug app forticron 2
#diag debug enable

                             <----- Add new interface widget.
Output will be:
Admin table has changed, updating interfaces for traffic history.
update_tr_history_intfs()-253: Maximum # of monitored interfaces reached. Failed to track new interface.
Disable the debug: # diag debug disable.
Also, CLI may yield below error while adding interfaces for monitoring bandwidth.
FGT#config system interface
FGT(interface)#edit <interface name>
FGT(<interface name>)# set monitor-bandwidth enable 
Cannot monitor this interface, already at max
node_check_object fail! for monitor-bandwidth enable
value parse error before 'enable'
Command fail. Return code -4 (reached the maximum number of entries)
FGT(<interface name>)#end