I have a GSM Modem SIM 900D. I am using it with my server and Python code to send and receive messages. I want to know how many Text SMS I could send and receive through this GSM modem per minute.
If you consider all protocols involved, including radio part, 300+ messages across a good dozen of protocols would have to be sent in order to deliver outgoing SMS to SMSC, and a great deal of waiting and synchronization will have to be involved. This (high overhead) will be your limiting factor and you would probably get 10-15 SMS per minute or so. 
Reducing overhead is only possible with a different connectivity methods, mostly to eliminate radio part and mobility management protocols. Usual methods are: connecting to a dedicated SMS gateway provider via whatever protocol they fancy, or acting as SMSC yourself and connecting to SS7 network directly.
GSM 900D Module Limit for Text Messages Sending & Receiving