A total of 386 extra customs officers will be needed in the event of a no-deal Brexit. As the UK would no longer be a member of the customs union goods entering Belgium from or leaving Belgium for the UK would be subject to customs checks at their port of entry/exit.
Even in the event of a deal that averts a hard Brexit the Belgian customs will still be able to find work for the new recruits. While some would-be customs officers have already been recruited and are currently been trained a further extra 200 customs officers still need to be found.
The Port of Zeebrugge
The most visible of the new recruits will be deployed to carry out checks on goods entry Belgium from the UK. Many of the goods entering Belgium directly from Britain do so through Zeebrugge docks in West Flanders.
As a major container port with goods arriving from and departing for countries the world over the customs at Zeebrugge are used to dealing with goods from countries outside the EU and the European Economic area. The UK will be treated in the same way as any other country outside the customs union.
The issue at Zeebrugge is that the Brexit will mean a large increase in the volume of goods that need to be checked. At Zeebrugge between 45% and 50% of goods are either going to or coming from the UK. Consequently, the UK leaving the customs union will lead to a big increase in workload for customs officers at the port.