limit number of buckets to 500k (#15668)

500k is a reasonable limit for any single MinIO
cluster deployment, in future we may increase this
value.

However for now we are going to keep this limit.

cmd/api-errors.go

@@ -176,6 +176,7 @@ const (
 	ErrBucketAlreadyOwnedByYou
 	ErrInvalidDuration
 	ErrBucketAlreadyExists
+	ErrTooManyBuckets
 	ErrMetadataTooLarge
 	ErrUnsupportedMetadata
 	ErrMaximumExpires
@@ -684,6 +685,11 @@ var errorCodes = errorCodeMap{
 		Description:    "The authorization mechanism you have provided is not supported. Please use AWS4-HMAC-SHA256.",
 		HTTPStatusCode: http.StatusBadRequest,
 	},
+	ErrTooManyBuckets: {
+		Code:           "TooManyBuckets",
+		Description:    "You have attempted to create more buckets than allowed",
+		HTTPStatusCode: http.StatusBadRequest,
+	},
 	ErrBucketNotEmpty: {
 		Code:           "BucketNotEmpty",
 		Description:    "The bucket you tried to delete is not empty",