Every fleet will always count as one attack. No matter when they reach or when/if other troops reach the same destination or/and at the same time
Only one thing:
When you (and/or somebody else) send two attacks which will reach destination at the same
second you cant be absolutely sure, which of them will attack first, no matter which of them is sent first (this counts for every browsergame).
The reason for that is, that mostly there is a timestamp-column in databases which is saved and/or read in seconds.
So its kind of "random" which of this "attack-entrys" (which are sorted by time) are considered first (surely, this depends on
how attacks are saved in the database and how the script reads this informations).
Please correct me, if Im wrong.