Total of 250 characters for all 5 fields. Does not matter if you cram into 1 field or spread across all 5. Here’s more info from Amazon’s help page…

Key Guidelines:

Amazon launched a feature that limits the length of the generic keywords attribute to less than 200 bytes in India, 500 bytes in Japan and 250 bytes in every other marketplace except China. The limits have been shown to improve the quality of search results. It applies to newly registered and existing ASINs.

Keep content within the prescribed length limit (less than 250, 200 for India, 500 for Japan):

• Length limit applies to total content in all generic keyword fields (a max. of 5 attributes).
• Whole entry will be rejected upon exceeding limit.
• Number of bytes equals number of characters for alphanumeric characters (e.g. a-z, 0-9) while other characters can be 2 bytes or more. Examples include ä (2 bytes), £ (2 bytes), € (3 bytes) or (3 bytes).
• Spaces and punctuation (";" “,”, “.”) do not contribute to the length limit, but words should be space-separated. Punctuation between words is unnecessary.

Optimizing keyword content for search discoverability:

• Do not include keywords that are not descriptive of the product.
• Do not include brand names (even your own) or other product identifiers.
• Do not duplicate content present in other attributes, such as title and bullet points.
• No need to repeat keywords; once is enough.
• Use keywords that are synonyms, hypernyms or spelling variations of content in visible attributes (e.g. if product title is ‘whiskey’, use ‘whisky’ in generic keywords).

For further information, see https://sellercentral.amazon.com/gp/help/10471