Kanban is about visualising a workflow. It helps teams to synchronise their activities and to improve their process.
Your question comes down to how best to visualise your current setup.
Advantages of a single board:
- A single view of all workflows
- It may highlight contention for resources between projects / systems
- Unrelated activities may be a distraction
- A complicated and cluttered board may be more difficult to interpret
The clearest scenario would be if you have teams dedicated to each project / system.
Team A works on Project X
Team B works on Project Y
In this case a dedicated board per team makes sense as there is little or no synchronisation between the teams.
If, however, you have one team that works on multiple projects / systems then it may make more sense to have just one board. This would help you to coordinate the activities of the team members and to spot any contention for resources.