50ml should be enough if your restoring faded areas.
BTW, I only do leather restoration as a full time job, so don't take any notice of me lol. First of all never put a conditioner on before 'dyeing'. Technically it's not a dye - it is a pigment, but I give up explaining why.
Clean with a leather cleaner if you can then leave to dry as the moisture would have absorbed in to the the non coloured parts. Then clean with a good solvent - I would always recommend alcohol (isopropyl) its dirt cheap off the bay. This will remove more grime and make it more acceptable for the pigment to stick properly. If you don't have existing colour loss then you can skip the next 'sealing' process.
Then I would use an aerosol lacquer (leather one of course), again cheap off of a leather supplier (you can either get a matt, satin or gloss). Spray this on the steering wheel to seal in and create a hard surface as opposed to the possible existing absorbent surface (that may still harbour grease - which won't allow proper adhesion). Then apply the pigment by either sponge, sprayer, micro roller or brush. I wouldn't use a brush myself as it will leave faint brush marks. When applied speed dry with a hair dryer then apply another coat until it is uniform.
Lastly I would apply the lacquer to the finished article to seal in the colour and make it hard wearing. This would do the job perfectly but I am a perfectionist so I would also use a PU water based lacquer on top of the aerosol one to obtain a true matt finish.
Good luck.
p.s. going back to the conditioner, it is an irrelevant product on these pigment finishes, it doesn't condition it at all as you have a lacquer coat. Conditioners are for aniline dyed leathers (actual dye as opposed to pigment) as they can dry out. However, leather retailers love to sell it because people think they need it
