、。
I've never seen this many people photograph the same rainbow.
Literal
Same rainbow [object-を], this-much many [genitive-の] people [subject-が] photographed ones [object-を] seen [experience-たことがありません].
A double-nested construction. The outer frame is ~を見たことがない ('have never seen'), and its object is これだけたくさんの人が撮影したもの ('things photographed by this many people'). 同じ虹を is fronted for emphasis — the shared subject of everyone's photos. これだけ ('this much / to this extent') emphasizes the unprecedented scale. 撮影した is a formal alternative to 撮った ('photographed'). The sentence expresses genuine amazement at collective attention to a natural phenomenon.