Hi,
The real answer is .... It depends.
It really depends on what sort of service you want for your domains ( IE speed of loading, up time etc ).
The actual limiting factor in domain hosting is not webspace or data transfer ( traffic ) but rather the number of domains on a server.
The reason is mainly down to the number of processes that are at work for each domain ( especially during the log rotation ).
Unfortunately during the last couple of years there are now many hosts ( mostly individuals working part time ) that can lease out a server and give people ' unlimited ' anything.
I have seen a dramatic increase in unlimited webspace / bandwidth / data transfer / domains etc. Talk to anyone that really manages servers and they will tell you that these claims are basically impossible.
It would be like going to your local garage and getting unlimited petrol ( gas ) for a low fixed fee a month.
Although things like cpu load / data transfer / bandwidth / webspace are not really ' physical ' products they all still cost money.
If someone is offering you hosting for 20 domains at $3 a month ( even if it is low transfer / webspace ) then you have to think if your business is worth more.
For a lot of people that will work out fine, just remember that most of these ' companies ' fill a server until it overloads then just go bust. They are playing the percentage game as we call it.
We do set a monthly data transfer limit ( my standards now is very small at 2Gb month ), but that is just to keep us in control. In four years we have only charged one company for over use even through quite a few do.
If you have a company in mind then just put their name into a search engine like Google and you will turn up opinions.
By the way do not take this as an advert for two reasons.
1) We only do one domain for one account.
2) I am selling up and getting out of the hosting business at last
All the best
Richard.