The sensor data collection problem using data mules has been studied fairly extensively in the literature. However, in most of these studies, while the mule is mobile, all sensors are stationary. The objective of most of these studies is to minimize the time needed by the mule to collect data from all the sensors and return to the data collection point from where it embarked on its data collection journey. The problem studied in this paper has two major differences with these earlier studies. First, in this study we assume that both the mule as well as the sensors are mobile. Second, we do not attempt to minimize the data collection time. Instead, we minimize the number of mules that will be needed to collect data from all the sensors, subject to the constraint that the data collection process has to be completed within some pre-specified time. We show that the mule minimization problem is NP-Complete and analyze the problem in two settings. We provide solutions to the problem in both settings by first transforming the problem to a generalized version of the minimum flow problem in a network, and then solving it optimally using Integer Linear Programming. Finally, we evaluate our algorithms through experiments and present our results.