Whenever a land you control enters, each landfall ability of the permanents you control will trigger. You can put them   on the stack in any order. The last ability you put on the stack will be the first one to resolve (As a result, you can have those abilities resolve in the order of your choosing.).

A landfall ability doesn't trigger if a permanent already on the battlefield becomes a land.

If Mossborn Hydra somehow enters at the same time as one or more other lands you control, its last ability triggers for each of those lands.

A landfall ability triggers whenever a land you control enters for any reason. It triggers whenever you play a land, as well as whenever a spell or ability puts a land onto the battlefield under your control.

To double the number of +1/+1 counters on Mossborn Hydra, put a number of +1/+1 counters on it equal to the number it already has. Other cards that interact with putting counters on it will interact with this effect accordingly.