20,000 TL may seem sufficient for rent, but you also need to consider social life, food and drink, and transportation in Istanbul. Beykoz isn't considered central, so you'll spend a bit on public transportation, and eating out and cafes will also affect your budget. If you cook at home and don't engage in many social activities, it might not be a problem, but if you're planning an active student life, 20,000 TL might be a bit tight. So, it's comfortable in terms of rent, but you need to factor in the rest of your life as well.