In CV4, it's 10 enemies/candles destroyed with the subweapon means the next small heart candle or the next random drop from an enemy becomes a DoubleShot. Same with the Triple Shot.
For the whip upgrades, if you're just starting (or recently died), then the first small heart candle becomes your first Whip Upgrade, and afterwards, you have to kill either three or five enemies/candles and then the next enemy drop or the next small heart candle becomes the second Whip Upgrade.
This is exactly the same rule for Alucard's Ball of Destruction magic upgrades.
EDIT:
Also, in the later games, if a candle is supposed to have a subweapon you already have, that candle instead has a large heart.
However, in CV1 and 3, it has a small heart instead (ripoff!).

This is also true if a breakable wall is supposed to have a double/triple shot but you already have your double/triple shot (cv1 has a small heart, cv3 has a large heart).
Similarly, there are some breakable walls that are only supposed to have large hearts, don't know why (just to be cool?).
You also have to calculate the alternate subweapon if you have a partner.
Trevor: Dagger, Axe, HolyWater, CrossBoomerang, PocketWatch
Grant: Dagger, Axe, Dagger, Axe, Pocketwatch
Sypha: Flames Book, Flames Book, Ice Book, Thunder Book, Pocketwatch
Alucard: small heart, small heart, small heart, small heart, Pocketwatch (ripoff!)