Minor correction OP; The pre Taliban government under Burhanuddin Rabbani lost Kabul in September 1996. The Taliban only emerged as a coherent force in 1994, in the south with their capital at Kandahar. April 1992 saw the mujaheddin triumph over the government of Najibullah, but the Taliban did not exist as we know them today. Fighting between the pre-Taliban government and the Taliban would continue well past 1996.

However, than said I can begin to go about helping you with your question. I'm not an expert, but I've finished a few books on Afghanistan in the past month, and I can help you with context and a few numbers.

It is important to state that Taliban numbers were never constant. The organisation was in a constant state of warfare from it's inception to past 1996 (Where the 20 year rule forces me to cut off). The lack of formal bureaucracy, and the propensity of the Taliban to talk itself up (when they talked at all), makes exact figures almost impossible to come by.

Rashid states that

"The regular Taliban army has never numbered more than 25,000-30,000 men, though these numbers could be rapidly increased before new offensives. At the same time Pakistani Madrassa students, who by 1999 made up some 30% of the Talibans millitary manpower, also served fro short periods of before returning home and sending back fresh recruits"

The above quote refers to the organisation from 1994-2000.

The book is light on numbers (due to the lack of record keeping by the Taliban), but he mentions 25,000 men mobilised (many of them madrassa students) in August 1995, and then was when the Taliban controlled less than half the country.

As to the scope of the Taliban military, they received very significant support from the government of Pakistan. This support helped them significantly, and they used were able to assemble forces capable of taking control of much of the country in conventional battles. As mentioned before, they recruited heavily from the religious schools (madrassas) in Pakistan and in general from the Afghan diaspora in Pakistan generated by the decades of warfare in Afghanistan. They also received money, assistance from Inter Services Intelligence Officers, and technical supports; Pakistani technicians were invaluable in maintaining the Taliban Soviet-era jet planes. By 1996 the Taliban were able to use tanks, artillery and infantry in coordination, although Rashid notes that the attacks often lacked sophistication, they could be devastating if they achieved surprise. As an example, Kabul fell without a fight after a surprise series of attacks on Jalalaba and then Sarobi forced the government to withdraw. Prior to this, Massoud, the government commander, was able to hold the city and repel attacks, but not push the Taliban back, with 25,000 men.

So in summary, the Taliban were able to muster large forces at times of need, but poor record keeping and the informal organisation of said forces leaves me at a loss to give exact numbers. Their manpower and their materiel support was substantial, and allowed them to recover from heavy losses.

Thanks for reading!

Sources

Taliban By Ahmed Rashid. An excellent book written by a Pakistani journalist with the first edition being published in 2000. I'm using the 4th (2010) edition. If you have any interest whatsoever in the Taliban, then I cannot recommend the book enough.

Ghost Wars The secret history of the CIA, Afghanistan and Bin Laden, from the Soviet Invasion to September 10, 2001 By Steve Coll. This book is centres on Al Qaeda and the American involvement (which was virtually nil in Afghanistan in the early 90s) and is used here to provide background information. (Again, an excellent book)