If the number of players is not a multiple of 2 (i.e., not 4, 8, 16, 32, 64, etc.), then go up to the next higher multiple of 2. The number of byes is that next higher multiple of 2 minus the actual number of players.
Examples:
• 9 players. Next-higher multiple of 2 is 16. Number of first-round byes is 16 - 9 = 7. So the number of first-round players is 9 - 7 = 2, and the number of first-round matches is 2 ÷ 2 = 1. The winner of that match will join the 7 byes to create a full winners'-side next round of 8 players.
• 19 players. Next-higher multiple of 2 is 32. Number of first-round byes is 32 - 19 = 13. So the number of first-round players is 19 - 13 = 6, and the number of first-round matches is 6 ÷ 2 = 3. The winners from those 3 matches will join the 13 byes to create a full winners'-side next round of 16 players.
• 87 players. Next-higher multiple of 2 is 128. Number of first-round byes is 128 - 87 = 41. So the number of first-round players is 87 - 41 = 46, and the number of first-round matches is 46 ÷ 2 = 23. The winners from those 23 matches will join the 41 byes to create a full winners'-side next round of 64 players.