When using Nihilego GX's "Symbiont GX" attack, can you place more prizes than your opponent started the game with?
Yes, you can. The prize pile is not limited to any maximum amount.
Source: TPCi Rules Team (2017-11-02)

Relevant cards

Note: All applicable cards may not be listed below.
SM4_103
SM4_114
SM4_49
SMA_SV62
SMA_SV62