View attachment 180445
As you can see in the chart above, the Artillery Tower spawns one landmine every 4 seconds at level IV. However, every landmine has a certain amount of time it is active. I don't know the exact amount of time after which it disappears. But for example, if it is 20 seconds the Artillery tower would spawn 5 mines, after that one disappears, then it spawns one again and one second after the second landmine the artillery spawned disappears again.
It could be that there is a maximum of landmines. So, if you would have 15 towers and a lot of landmines it doesn't spawn landmines unless there disappears one again.
Did this clarify it a bit? If not, I'll try my best to explain it better.