Microsoft Exchange Standard vs. Enterprise

I am researching MS Exchange server as an upgrade to our existing email server and in reading the documentation between Standard and Enterprise it says that Standard supports up to 5 email databases and Enterprise up to 100. Our company utilizes about 50 email accounts at the moment and I wanted to know if each "email database" is comprised of a specific domain and/or how many email accounts are available in each email database. Any enlightenment on the terminology between the 1-5 databases and their relationship to actual email accounts would be helpful.