How many lure modules can a trainer drop simultaneously?
Solution 1:
A player can drop as many lures at any simultaneous moment as they have in their inventory (and thus use them), however - A PokéStop can only have one lure module at a time.
Since a PokéStop can only have one lure at a time, the next person to deploy a lure will have to wait until the current one runs out before being allowed to use his.
tl;dr Deploy away!