It's based on the Japanese numbers of the SIR Mega ex cards. As well as the total amount of SIR cards announced in the English reveal.
The English announcement mentioned 10 SIR cards. And we have the Japanese numbers of:
Mega Brave:
87. Mega Venusaur ex
88. Mega Lucario ex
89. Mega Absol/Mawile ex
Mega Symphonia:
87. Mega Gardevoir ex
88. Mega Latias ex (based on it being the only Mega being between Gardevoir and Kangaskhan)
89. Mega Kangaskhan ex
As Kangaskhan is the final ex of the set, there can't be any ex SIR cards after that one in Mega Symphonia, meaning that it is likely for Mega Brave to also only feature 3 SIR ex cards. As the English announcement mentioned 10 SIR cards, and with 6 SIR ex cards, it seems almost confirmed that the remaining 4 SIR cards will be the 4 Supporter cards revealed for the set.