。
I bought another sweater, so now I have 21.
Literal
Different sweater [object-を] bought [because-ので] currently 21 become.
別のセーターを買った triggers the count update: now totaling 21. The ので connects cause to result. ~になりました marks the change of state — 'it has become 21.' A sentence that's more interesting for its implied obsession (21 sweaters!) than its grammar.