A default UI of the control is designed so as to fit as many months as can fit in the current control size, taking into account the size of each single month calendar which is determined by the MonthWidth and MonthHeight properties. As a result, when control is resized, the actual number of displayed months can be lower or equal to the MonthCount property value.
You can change default behavior by using custom ItemsPanelTemplate. For more details see MonthsPanel property description.