Hi Naserzadeh,
Because you have only 50 users in the environment, so we suggest you deploy SFB standard edition. I will share a document about how to deploying SFB standard edition
https://insidemstech.com/2015/06/30/step-by-step-skype-for-business-server-standard-edition-deployment/
Recommended hardware for Front End Servers, Back End Servers, Standard Edition servers, and Persistent Chat Servers:

Please refer to the document below
https://technet.microsoft.com/en-us/library/dn951388.aspx
Note: Microsoft is providing this information as a convenience to you. The sites are not controlled by Microsoft. Microsoft cannot make any representations regarding the quality, safety, or suitability of any software or information found there. Please make
sure that you completely understand the risk before retrieving any suggestions from the above link.
Best Regards,
Alice Wang
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