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.
In Theravada, the word 'Bodhisatta' refers to an unenlightened person seeking Buddhahood.
In other words, a Bodhisattva is not a Buddha in Theravada.
Bhikkhus, before my enlightenment, while I was still only an unenlightened Bodhisatta...
This is why most Theravada reject the Mahayana doctrine of Bodhisattva Vehicle and this is why there are two separate doctrines (Theravada & Mahayana) which despise each other. Theravada views the Mahayana doctrine as superstition and Mahayana views the Theravada doctrine as "hina", which means "inferior, low; poor, miserable; vile, base, abject, contemptible, despicable".
Also, in Theravada, a Buddha is an Arahant but an Arahant is generally not regarded to be a Buddha. Therefore, Theravadans generally aim for Arahant and keep the name 'Buddha' reserved for Gotama Buddha.
The difference between Arhantship and Buddha is that the Buddha rediscovers the meditative technique through his/her own effort (rediscovers the technique since the technique is lost as it has been a long time since the last Buddha) that leads to liberation while the arhat learns the technique through Buddha or a line of teachers that have preserved the technique in its purity since the last Buddha. They both are enlightened beings.
I cannot say if it is common or uncommon to aspire to be a Buddha. To aspire for liberation is certainly a commmon goal.
"Bodhisatta" in your question is a word mean "a man who enlightened by himself, then help uncountable others to enlighten follow him too".
According above meaning, there is very less person, uncommon, who going to be a Buddha because almost everyone hates suffering, loves happiness. Helping others are suffering. And if one chooses to help uncountable people, it comes with uncountable suffering, too, because each one comes with individual complex problems.
Most of people will not be Bodhisatta because they will give up to that uncountable suffering.
Every Bodhisatta borns with uncountable jobs, he has to plan for every step from current life to the next and next and uncountable next to the last life. Bodhisatta must work hard to practice people for making sure that people will birth with him and practice to enlighten follow him in his last life. He has only one chance to help people because it is his last life, so he has to work very hard to collect the actual practicable people before his last life.
The fail Bodhisatta is called "PaccekaBodhisatta" because he can't let the others enlighten follow him in his last life.
And that's why the Buddha praises PaccekaBuddha more than other Arahanta. Sāriputta and Moggallāna practiced to be the chief male disciples of Gautama Buddha only for 1 asaṇkheyya and 100 thousand mahākappa after making a decision in front of the Buddha, but PaccekaBuddha practiced for 2 asaṇkheyya and 100 thousand mahākappa to enlighten in the last life. All of them can enlighten at the time of making decision, but they didn't because they want to help people.
How does PaccekaBuddha help people? And why his helping is better than Sāriputta and Moggallāna?
PaccekaBuddha help people by helping many other Bodhisattas. All kinds of Bodhisatta help each other to collect people. You can see the co-working teamwork in Jataka-Atthakathā and some other Atthakatha. Who practice longer is who can help greater.
That's why PaccekaBuddha is better than Sāriputta and Moggallāna.
PaccekaBuddha can't help people to enlighten in his last life, but he can help many other Bodhisattas to let uncountable people enlighten follow more than Sāriputta and Moggallāna because PaccekaBuddha practiced in the longer period.
The only fail of PaccekaBuddha is "they can let people enlighten in their last life".