The original intention was that the UP shall cover ALL of the 27 EU Member Countries. This may still be the case in the future. 

However, as of today, the 17 countries participating in the UPC system (when it starts) are: Austria, Belgium, Bulgaria, Denmark, Estonia, Finland, France, Germany, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Portugal, Slovenia and Sweden.

7 countries which may still join the UPC system are: Cyprus, Czech Republic, Greece, Hungary, Ireland, Romania, and Slovakia.

Three countries, namely, Spain, Poland and Croatia are currently not participating in the UPC system.

Due to Brexit, the UK has left the EU. Consequently, the UPC system shall not cover the UK (one of the top three countries in which EP patents are validated).

The EPC has 38 member countries, including 11 non-EU countries. Those non-EU countries, namely, Albania, Iceland, Switzerland, Lichtenstein, North Macedonia, Monaco, Norway, San Marino, Serbia, and Turkey, and the UK are not covered by the UPC system. Patent protection for these countries may only be achieved via the traditional route (classical EP or national route).