Minecraft Java Edition, Commands – How to Count Total Items on the Ground

I didn't run the command, i just read the OOC and guessing the the principle, hope that i am right.

The command is just teleport the player ~ ~-1 ~ first, so that the player is in the ground. Then, it is teleporting the player ~.00002 and ~-0.00002 in x and z direction repeatedly(positive for 1 time, in the next tick negative...), thus the player can pass through the border between each block.

You can summon the melon slice at the player's position in item form (with a small pickup delay) and then use /data to modify the Count property

To summon the item:

/execute at @a[nbt={SelectedItem:{id:"minecraft:villager_spawn_egg"}}] run summon item ~ ~ ~ {PickupDelay:10,Item:{id:"minecraft:melon_slice",Count:1,tag:{CustomModelData:10000101,display:{Name:"{\"text\":\"Blue Melon Slice\",\"italic\":\"false\"}"}}}}

to modify the Count property:

/execute as @a[nbt={SelectedItem:{id:"minecraft:villager_spawn_egg"}}] at @s store result entity @e[type=minecraft:item,limit=1,sort=nearest,distance=..0.2] Item.Count int 1 run scoreboard players get @s <objective_with_count_data>

and then clear the spawn egg from the player's inventory. There's the concept.