^ The total sum is 200% because each currency trade always involves a currency pair; one currency is sold (e.g. US$) and another bought (€). Therefore each trade is counted twice, once under the sold currency ($) and once under the bought currency (€). The percentages above are the percent of trades involving that currency regardless of whether it is bought or sold, e.g. the U.S. Dollar is bought or sold in 88% of all trades, whereas the Euro is bought or sold 32% of the time.
This depends on how liquid the currency is, or how much of it is being bought and sold at any one time. The most liquid currency pairs are the ones with the most supply and demand in the Forex market, and this supply and demand is generated by banks, businesses, importers and exporters, and traders. Major currency pairs tend to be the most liquid, with the EUR/USD currency pair moving by 90-120 pips on an average day.
By contrast, if you just traded 20 EUR, a loss would not significantly affect your account balance. It would provide you with the opportunity to learn from your experience and plan your next trade more effectively. With this in mind, limiting the capital you are prepared to risk to 5% of your account balance (or lower) will put you in a better position to continue trading Forex (and improving your technique) over the long term.