Left and rightclick into the diagram on top. If you click on the blue moves in the bottom half it will execute the moves. If you open a database, then open a game from it and then open the eco browser then it will show you how many games each eco entry has in the database.
#6 @Breaking-Limits That would depend on how many moves you count as "opening". If you have Stockfish available, you can let it calculate that by running the executable and executing the command "go perft 1" (without quotes, and replace the 1 by how many plies (half moves) deep you want to go). This exact command will return "Nodes searched: 20" at the end, because there are 20 possible first moves. If you run "go perft 2", you'll get 400, because there are 20 possible first moves with each 20 possible responsed. "go perft 3", "go perft 4", etc. all go deeper. "go perft 6", for example, returns 119060324 - that's a lot if you realize we're only 3 full moves in the game!