What is a reasonable range of number of people that could participate in a cohesive and centrally organized migration/encampment?
Ancient times, desert terrain. People include adults of all ages, children of all ages, and large numbers of livestock.
Food is not a problem. Most of the provided food does not need to be cooked (storage not an issue) and for the food that does need cooking, they will all have some basic cooking equipment and all the dried out animal manure they want (a common source of fuel).
Water is not a problem. There are short periods at the start of the journey with no potable water but those issues are addressed and solved.
Most everyone is walking. There are donkey and oxen for pack animals and to allow people who are very elderly, ill, or injured to ride or maybe be pulled. Small children can be carried or helped per above. Few or no horses. They have tents to sleep in.
There are really good roads for some of the journey though in many places they will avoid roads, or at least the more popular ones. Some of the terrain is mountainous. All is hot and dry.
They are safe for the first few days; after that, pursuers come after them. So they are moving briskly but not at a punishing pace.
They are choosing isolated areas but basically the region is populated and/or well-traveled. News about them gets around. They have some skirmishes with annoyed locals (one big battle). And it is possible they are able to do some trading or purchasing of goods.
Moving happens in stretches as short as one day and as long as a few days. Encampment happens in stretches as short as one night and as long as several months, though most are not that long.
They are traveling as one group and need to make joint decisions and share information and resources.