It is not possible to accurately know the number of people eligible to apply to the EU Settlement Scheme, hence it is impossible to know how many people still need to apply. The most up to date population figures from year ending June 2020 show Merton having 23,000 EUSS eligible residents, still, 35,50 applications to the EU Settlement Scheme have been received from Merton residents according to Home Office statistics up to September 2020.
There are several reasons why we cannot compare numbers of people granted status with official EU citizen population estimates. For example, Home Office counts the applications from individuals twice if they receive pre-settled status and then convert to settled status, or if they re-apply after an initial application is not granted. Furthermore, the resident data sources that we rely on are outdated and have important limitations.
This report from the Migration Observatory on understanding the EUSS data goes into more details: https://migrationobservatory.ox.ac.uk/wp-content/uploads/2020/06/Report_Not-Settled-Yet-Understanding-the-EU-Settlement-Scheme-using-the-Available-Data.pdf