seems people sometimes dont want to help, there are a lot of good ways and a few plugins that do what you want, here is a list of a few , mode2x2, remode 2x2, mapbalance by blacksignature, and a few others...
i don`t think that he wants players walking around the map with low numbers of online players, so just not allowing to plant B won`t help, walkguard doesn`t work with numbers of online players
you don`t even need a plugin, here is a version of a dust2 that will open a gate depending on a number of online players
donwload link here since for some reason uploading zip files aren`t working on the moment
all the best