With the function below, dropping the last tnt gives a new one, but placing it gives an invisble one instead (right click with the slot it would be in places is too, picking another one up from the floor reveals the second one, etc.)

Running in minecraft:tick:

execute as @a[team=blue,nbt=!{Inventory:[{id:"minecraft:tnt"}]}] if score Aqua tnt matches 1.. if score match match matches 2 run function tntbattle:tnt-blue

The referred tntbattle:tnt-blue function:

give @s minecraft:tnt
scoreboard players remove Aqua tnt 1