Well it really depends, You could have many servers connected. But what really matters is the amount of hits and services running. My old school had a 50mb connection and they had over 500 computers and there was very little latency.
If you are looking at offering hosting services, you can look at it this way. A 45Mbps connection fully saturated will allow you to transfer more than 14,500GB of data in a months time period. Consider that most hosting providers offer from 100Gb to 1000GB or more of data transfer with their packages, that may help you figure out what you want to know.
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As above, you need to calculate your service offerings versus what you are able to provide. Almost everyone offering x000GB of data transfer a month are usually overselling their networks somewhat.
Likewise, if you decide to go down a fixed bandwidth option of say offering guaranteed 2Mbps to each client, then it makes your calculations a lot easier but not necessarily the most cost effective unless you oversell in the hope that your users won't actually use your services.
Your business plan should support whether you have selected the right type and amount of bandwidth - one other thing I would consider is that you say a single circuit, are you not planning any resilience in your connectivity?
It depends on what your servers will be used to, and how much traffic will it pass through them. It sounds decent for a very small data center, I guess.