One 50w cob should be able to grow one plant with out a problem. 2 50w cobs will do even better. If your goal is high yeild, then use as much light as possible. With cobs its not reallt about "watts per cob".. Its more about how efficient they are running. A 100w cob (Depending on model and manufacture recommendations) is less efficeint than a 50w cob. Which means less usable light. With cobs, the lower the watts, the more effeinct the cob is. Most have great efficiency at around 50w. I would definately do two 50w over one cob running at 100w.
@bob142g has an amazing example of this in his journal..
https://www.autoflower.org/threads/blue-mystic-under-bigsmos-cob-lights.58800/page-5#post-1455321
He started with one 55w [HASHTAG]#Autocob[/HASHTAG] and later added a second 55w [HASHTAG]#Autocob[/HASHTAG] for a one plant grow... And he grew a flippin TREE! Lol. So have a look around at a few journals in the Cobshop.net thread or in the DIY lighting thread and you will find plent of examples. I personally run two 55w [HASHTAG]#Autocobs[/HASHTAG] for three plants in a 3x3. Just got a third [HASHTAG]#Autocob[/HASHTAG] this week so I can run one more plant. So 3 cobs.. 4 plants (two 3gal, two 5 gal) in a 3x3 and it definitely enough. Lights are ran 24/7 and height depends on stage.. For a small grow tent, like a 2x2 or 3x3.. Just hang it at the top and let the plants grow to the light. Mine are kept at the top of my tent.. Hope this helps a tad and welcome to AFN!