He's seemingly modeling his crew after Whitebeards. So 10 Titanic commanders should each have 100 men making for a main crew size of about 1000. Like WB, it's his allies which make up for his numbers. We know he has direct allies like Peachbeard, but the amount is hard to say.
The pirates at Hachinosu are numerous, but with it being more of a "Pirate Island" not all are closely allied with him. 5000-10000 range is what I predict as of now.