I am trying to understand how many horses Mr Bennet had. I take it there were only two horses to pull the carriage(or help with the farmwork), but, provided the one Jane used(Nellie?) was still in the stables at Netherfield, was there yet another riding horse when Elizabeth was planning her visit with Jane?

Elizabeth, feeling really anxious, was determined to go to her, though the carriage was not to be had; and as she was no horsewoman, walking was her only alternative.
the bold words suggest the availability of another horse.