起码 indicates the minimum or very least. In Chinese, it is used both as an adjective before nouns (e.g., 起码的要求 'the minimum requirement') and — very commonly in spoken Chinese — as an adverb before verbs or quantities to mean 'at least' (e.g., 起码有三个 'there are at least three'). Although the part of speech here is adjective, the adverbial use is widespread and natural.