No, there is no limit.
When you create a COBOL trigger within Lawson it allows you to do unlimited, you just called the add variable api for each 10 variables you want to add before calling the Release api.
calling the package com.lawson.bpm.eai.trigger should work the same way
You can have as many variables as you want and they don't even have to be defined in the Service Variable definition, that is used only by the Designer tool to show what workunit variables are available in dropdowns for Assign's etc. Does not mean you can not reference or use a workunit variable you added to a trigger but did not add to the Service Variable definition.
Keep in mind there is some overhead to having workunit variables. They are stored in a LOGAN table WFACTVAR and is updated with a record after EACH node is called within a flow, so if you have 20 workunit variables and the flow processes 15 nodes, that is 300 entries in that table for one execution of the flow. Any variables you add in the start node also write to this table.