‘And they forgot to take bread. And they had no more than one loaf in the boat with them.'
In spite of our natural curiosity we are not told who had forgotten to take the bread. Someone was responsible and had failed in their responsibility. Perhaps it was all of them, each leaving it to the other. But however that may be they had realised to their dismay that they were now entering Gentile territory with only one loaf between them.
So Jesus will now take the opportunity to draw their attention to the fact that they were not only short of bread, but also of the bread of truth. (This in preparation for the awakening soon to come at Caesarea Philippi and its high mountain (Mark 8:27 to Mark 9:8)).
In the context which has preceded this episode the disciples have been thwarted by only having five loaves and then seven loaves. Thus this description of their being down to ‘one loaf' might be intended to indicate that they have now reached the end of their resources. What they should of course have remembered was that the Syro-phoenician woman had only needed crumbs.