The best way to do this is with a set of digital scales that has grammes, just weigh 50, take a note of the weight and then follow that same weight all the time, you may get the odd one less or extra but its the quickest way.
It varies. Macanudo "Ascots" come 10 to a tin. There are quite a few brands that also package 10 to a tin or box. It also depends on the ring gauge of the product.