The number of people a full bus holds — so 'a busload of tourists' means roughly forty or fifty of them, arriving together. In practice English speakers rarely use it to count carefully; it is a rough, lively way of saying 'a whole big group of them, all at once'. That is why you often hear it with a slight sigh or a joke in it: 'A busload of students just walked into the restaurant.' You can also say 'busloads of' (plural) to mean a great many: 'busloads of visitors come every weekend.' It usually describes people, but you will sometimes see it used loosely for things carried on a bus. English builds many words this way — a carload, a truckload, a planeload, a boatload — and 'a load of' by itself can mean simply 'a lot of' in casual speech.