SIP trunks DO NOT require PVDMs to work, they're purely IP, there's no TDM conversion needed.
You have already asked a similar question before:
https://supportforums.cisco.com/discussion/12742321/dsp-resources
How many, is based on YOUR analysis of the network, requirements, used codecs (by telco, and your system), etc Once you have that info, you can use the DSP calculator.
If you're not familiar with this, I'd strongly suggest you get in touch with a Cisco Partner for assistance.
HTH
java
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