In my experience as account servant the congregation expenses were covered by very few people. Most contributed nothing - and that includes for the literature. In all my life as a JW the total I contributed over a 30 year period was less than $50.00
This was the same that I experienced. In a congregation of a little less than a hundred, we'd have a few hundred dollars donated each month, usually by the same handful of people, towards the worldwide work. I haven't been a servant since the days of the new mandatory donation, so I'm sure this number has drastically increased.