Updated below:
Reuters, Jan 5:
Iranian-backed Hamas is estimated to have about 25,000 fighters.
AP, Jan 5:
Hamas is believed to have some 20,000 gunmen ...
BBC, Jan 4:
The Hamas military wing - the Izz al-Din Qassam Brigades - is thought to have about 15,000 members.
Gulf News, Jan 4;
Hamas has several thousand fighters, a small proportion of the 1.5 million population of Gaza who are all now suffering a relentless assault.
NYT, Jan 4:
In recent days, most of those arriving at Shifa [hospital] appeared to be civilians. On Sunday, there was no trace here of the dozens of Hamas fighters that the Israeli military said its ground forces had hit in the past few hours in exchanges of fire.
NYT, Jan 4:
At Shifa, Gaza City’s main hospital, dozens of casualties seen being brought in over many hours all appeared to be civilians.
UPDATE:
To answer my own question: Hamas Has How Many Fighters?
In 2006 Hizbullah in Lebanon was said to have 600 to 1,000 active fighters and some 5,000 reservist. Given the size of Gaza I estimate that there are probably 200 to 400 active Hamas fighters in Gaza with less than 1,000 reservists and people in training.
The high numbers with the implausible variance given above by the media likely originate from Israeli sources and are highly exaggerated to let the enemy look big. That big number than justifies these terrible attacks on mosques during prayer time, ambulances and hospitals.
The small Hamas cadre can not fight the Israeli military. Currently at least three infantry/paratrooper brigades and a tank brigade are involved on Israel's side. Hamas will try to inflict some casualties on these, but it can never take a real stand against them.
There is still the question what Israel is aiming at here, except maybe kill lots of civilians. Do Livni/Barack/Olmert really have a plan? How is this, in their view, supposed to end?
--MORE--"