Contar campos não vazios mysql

Seu código tenta contar o número de linhas em que todos os campos não são nulos. Você deve estar usando is not null em vez de apenas not null .

Para contar o número de campos, use isto:

SELECT sum((listing_photo_1 IS NOT NULL) +
           (listing_photo_2 IS NOT NULL) +
           (listing_photo_3 IS NOT NULL) +
           (listing_photo_4 IS NOT NULL) +
           (listing_photo_5 IS NOT NULL) +
           (listing_photo_6 IS NOT NULL) +
           (listing_photo_7 IS NOT NULL) +
           (listing_photo_8 IS NOT NULL)
          ) as total
from listings
WHERE pmpid = '$pmpid';

Para contar o número de linhas:

SELECT count(*) as total
from listings
WHERE listing_photo_1 IS NOT NULL AND
      listing_photo_2 IS NOT NULL AND 
      listing_photo_3 IS NOT NULL AND 
      listing_photo_4 IS NOT NULL AND 
      listing_photo_5 IS NOT NULL AND 
      listing_photo_6 IS NOT NULL AND 
      listing_photo_7 IS NOT NULL AND 
      listing_photo_8 IS NOT NULL AND 
      pmpid = '$pmpid'";

EDITAR:

Se eles estiverem em branco, use a lógica como esta:

SELECT sum((listing_photo_1 IS NOT NULL and listing_photo_1 <> '') +
           (listing_photo_2 IS NOT NULL and listing_photo_2 <> '') +
           (listing_photo_3 IS NOT NULL and listing_photo_3 <> '') +
           (listing_photo_4 IS NOT NULL and listing_photo_4 <> '') +
           (listing_photo_5 IS NOT NULL and listing_photo_5 <> '') +
           (listing_photo_6 IS NOT NULL and listing_photo_6 <> '') +
           (listing_photo_7 IS NOT NULL and listing_photo_7 <> '') +
           (listing_photo_8 IS NOT NULL and listing_photo_8 <> '')
          ) as total
from listings
WHERE pmpid = '$pmpid';