Yes, but even depends on the CPU performance. When you generate a call this call use a codec, generally G711, but you can define your preferred codec like G729, this codec is more used when you use a Internet route to make your call. Every codec has a bandwidth consumption so if you have a internet dedicated of 2Mb you can generate 63 simultaneous call.
This is by: (2048 - 3% ) / 31.2, the 3% is the headers more or less that you consume in every call, and the 31.2 is the consumption for a call with g729 codec. Obviously you need a SIP provider.
When we talk about T1, here the maximums calls you can generate are the maximums channels in your T1 for the DSL applies the above answer. The CPU help to translate between codecs, so the performance will decreased if you use differents codecs in your call.
No, only one dynamic or static IP, but one sip provider.
Yes, you will need 40 differents land lines, for that number is better a E1,T1 or sip provider.