。
I found some wild mushrooms under the tree.
Literal
Tree [possessive-の] under [at-に] several [possessive-の] wild mushrooms [object-を] found.
いくつかの ('several, a few') is the vague quantifier for unspecified small counts, parallel to 何人か. いくつ is 'how many?' and the か makes it indefinite. The relative clause 'wild mushrooms that I found under the tree' is expressed as the object of 見つける ('to find'), not a modified noun.