Will a different serial be given for each 50 users or will they be using the same serial? 

the small site license uses only one license code for all seats. 
Please be sure, though, to also look into the restrictions that apply 
to the small site license:

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1) This license code may be used on up to 50 computers to which the 
legal entity this license code has been issued to has to hold all 
property and owning rights. 
2) This license code must not be used on computers you are providing 
to others for rent, sale, for leasing or comparable purposes. 
3) This license code has to be removed from a computer as soon as it 
stops meeting the previously outlined requirements. 
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Best regards, 
Felix Schwarz