Well they started off with a lot more than 15,000, but we don’t really know how many people there were at the beginning, or how many survived until the end.

Jonathan Riley-Smith compiled the numbers in various medieval and modern historians. The number of crusaders who left Europe in 1096 has been estimated at anywhere between 60,000 and 300,000. One medieval author said there were 100,000 knights and 200,000 infantry at the beginning, and by the time they conquered Jerusalem in 1099, there were only 1000 knights and 5000 foot soldiers. But that would mean 99% of the knights died along the way! The numbers given by medieval authors are usually unreliable, since they vastly overestimate the size of armies, as well as the number of people killed in battle.

I’ll stick with the numbers given by Thomas Asbridge, who wrote the standard history of the First Crusade (at least, it’s the one I use here the most often). According to Asbridge, the crusade started with 100,000 people. When they captured Antioch in 1098, there were only 30,000. When they arrived in Jerusalem, they had about 1,300 knights and 12,000 infantry. After taking Jerusalem, they faced an army from Egypt in August 1099. The crusader army had 1,200 knights and 9,000 infantry.

So, in fact there probably weren’t even 15,000 people left at the end, more like 10,000. And if there really were 100,000 at the beginning, that means 90% of them didn’t make it to the end. Even if there were only 60,000 at the beginning, that’s still 75% death rate (although not all of them died, some of them simply left the crusade and went back home).

But your question still remains - how did they feed all of these people along the way? At first everyone must have brought as much food and water as they could carry, but that wouldn’t last very long. There was no such thing as a supply train, especially not one that would stretch all the way to Jerusalem. Instead, the crusaders had to forage for food wherever they could. They mostly followed the Danube River, so while they were still in Europe they were never really far from water. They could get food from towns and villages along the way.

When they entered Hungary, and then the Byzantine Empire, they often tried to buy bulk amounts of food. The local rulers might attempt to set up a special market where everyone could buy food fairly. But sometimes this turned out to be impossible, and the crusaders would just pillage the land and take what they wanted. The Hungarian and Greek armies sometimes had to fight them to make them stop.

Once they reached Constantinople, and the Byzantine emperor sent them across the Bosporus into Anatolia, the crusaders had access to Byzantine administration and logistics. For awhile, they could be supplied by the Empire. But as they travelled further east, Byzantine supply lines could no longer reach them, and now they were in enemy Seljuk territory. There wasn’t enough food or water for everyone and lots of people died along the route. They could resupply again when they captured Antioch, but then they were besieged in Antioch by a Muslim army, and some crusaders starved there as well. On the march from Antioch to Jerusalem, they had the same problems, and many died of starvation then too.

Sometimes they resorted to eating donkeys or dogs or any other animals they brought with them. Crusaders with horses sometimes drank horse blood when they had no water. They even had to eat their horses if they were really desperate - but this was seen as something shocking, in a society where horses were the most important animal. At the siege of Ma’arrat (between Antioch and Jerusalem), it was reported that some crusaders were so hungry that they ate the bodies of dead Muslims! It’s possible that this is an exaggeration, but they definitely still had problems finding food and water even when they were close to Jerusalem.

So the answer is that no, there was never enough food or water. They bought food when they could, but sometimes they pillaged it. Sometimes they had to hunt or forage for food in the countryside. They took whatever they could find, wherever they could find it. But often, they couldn’t find anything, and they had to starve. The casualty rate was enormous, due to deaths in battle, or exposure to the heat and the cold, and some people gave up and returned home - but a large percentage of deaths were due to hunger and thirst, because they couldn’t reliably provision the entire army.

Sources:

Thomas Asbridge, The First Crusade: A New History (Oxford University Press, 2004)

Jonathan Riley-Smith, “Casualties and the number of knights on the First Crusade”, in Crusades, vol. 1 (2002)

John France, Victory in the East: A Military History of the First Crusade (Cambridge University Press, 1994)