To get the current database memory cache size, use the "Get database parameter" command. This command accepts a selector parameter and returns the function results. To obtain the memory cache size, pass in the constant "Database Cache Size" or the value 9. The command will return the current cache size in bytes. To obtain the cache size in kilobytes, divide the result by 1024.
For example:

C_Longint($cacheSize,$cacheSizeKilo)
C_Text($textCacheSize)

$cacheSize:=Get database parameter(Database Cache Size)
$cacheSizeKilo:=$cacheSize/1024
$textCacheSize:=String($cacheSizeKilo) + " Kb"

To find more information about the command go to https://www.4d.com/docs/CMU/CMU00643.HTM.