The number you are seeing at the top (below review title) refers to the unique number of articles after removing exact matches.
Exact matches refer to references from your uploaded searches that appear multiple times with the same exact content and hence were automatically deleted by Rayyan. All exact matches are grouped in a filter box called "Auto Deduplicated".
So, if Rayyan discovered 10 references with 2-exact matches, this means that the number of studies you will see will be equal to the number uploaded minus 10.
If Rayyan discovered 10 with 2-exact, and 4 with 3-exact matches, you will subtract 10x1 and also 4x2, and so on.
To calculate the number, you can sum the number of references in each upload then subtract exact matches, as explained above.