Are there Theravada practitioners that aspire to be Buddhas? As far as I know, most Theravada followers practice according to the Pali Canon even though they accept the Bodhisattva vehicle as well. So, the main focus is to attain Arahantship for most people. My question is: How common is it for Theravada practitioners to aspire to be Buddhas?
There are many who aspire to be Buddhas even in the Theravada community. But it is not something that is usually encouraged. It is left up to the individual choice. 

Those who make such aspirations out of ego and lack of understanding of the enormity of the task are discouraged and adviced to give it up. Those who  have mastered the Tipitaka at the highest level and proven their worthiness to the community are given blessings. Even then it's not sure until they actually get the blessings of another Sammasambuddha. And it's not certain if they ever get the chance to meet another Buddha, let alone getting the blessing. Those who never get that blessing end up being Pacceka-Buddhas after a long time, if they don't give up the pursuit.