Change column number of the single store page.

functions.php

<?php

function single_store_page_product_columns( $columns ){
  if( dokan_is_store_page() ){    
    return $columns = 4; //enter your desired number
  } else {
    return $columns;
  }
}
add_filter( 'loop_shop_columns', 'single_store_page_product_columns', 99);

?>