In the following use case the transition will only be displayed, if the current assignee doesn't have more than 10 issues that are in the same project and in status In Progress.
Configuration
Add the condition or the validator to the desired transition or create a new transition.
JQL
Enter the following expression:
project = "%{issue.project.key}" and assignee = "%{issue.assignee}" and status = "In Progress"
The field codes will be replaced by the actual field values of the current issue.
Condition
Choose Number of issues returned by the JQL query satisfies a logical expression
Expression
Enter the following expression:
{issue.temporaryNumber1} <= 10
This way the transition will only be displayed, if the above entered JQL query doesn't return more than 10 issues.
Error message
The current assignee is involved in too many issues in that project that are in status "In Progress".