That depends on the size of terrain you have available to play with. If playing with terrain sets built 10-30 years ago things like 12x12" pieces of area terrain are not uncommon; so a few of those with some scatter might be more than enough for gaming groups.
The event also has 'work with everything system' listed; AoF is 10-15 and GF is 15-20, so 10-20 it was. Also, with the objectives being terrain themselves, that's 4 more pieces right there.
On the objective being line of sight blocking, it is a gimmick- the mission should have some sort of interesting twist. However, I don't see these geysers as small 20-40mm rounds, but rather 3-4" rounds of crystals 6"+ tall. To get the feel across that these are massive pillars that appear to grow from the ground I had to give them a rule that implied that importance to the reader.
