First, let’s go through the history of his bounties
30,000,000
100,000,000
300,000,000
400,000,000
500,000,000
1,500,000,000
3,000,000,000
They all start with either 1 3 5 or the outlier which is 4, so with the next bounty probably starting with 1,3 or 5, I predict Luffy’s next bounty will be 5,000,000,000 berries