number two
貮 centers on the concept of the number two, serving as an alternative or variant form for writing 'two' in certain contexts.
貮 is a variant form of 弐, which itself is a formal or legal alternative to 二 (two). The exact historical development of this particular variant is uncertain, but it likely arose as a stylized or mistaken rendering of 弐.
The left side 貝 (shell) and the right side 弋 (a stake-like shape) together form a variant of 'two'. Imagine two shells tied to a stake, representing the number two.
For ニ, think of a knee (ニ) bent twice—once for each of the two shells tied to the stake. The knee sound cues ニ while the two shells reinforce the meaning of two.