I've recently decided to replace my Linux firewall/router with a more manageable appliance(better VPN, firewall, proxy). I've been searching the net for a good explanation of the significance of throughput and simultaneous connection as compared to the size of the internal network, but have been unsuccessful so far. I understand what the terms mean but I don't understand how they work as compared to the size of the network. The easiest way to ask it, if I have a smaller network, say 50 workstations, can I use a router with smaller numbers in these two areas?
Simultaneous connections generally refers to the number of specific IP addresses that are sending/receiving traffic through the firewall. That is to say, 1 PC with 3 browsers open, running Skype, and playing TF2 will count as 1 simultaneous connection. 20 PCs doing the same thing will count as 20.
Throughput refers to the aggregate amount of bandwidth being used by ALL connections. So if you have 1 PC using 10Mbps bandwidth, and your appliance supports a maximum of 5Mbps, you'll be throttled. If you have 2 PCs, using 20Mbps bandwidth, and the router supports 200Mbps throughput, you'll be fine.
As a general rule on throughput, look at the bandwidth you're getting from your ISP. If you have 3 T1s (4.5Mbps up/down), then as long as the router can handle more throughput, you're fine. I suspect since you're asking this question you're not in the hundreds of Mbps range where throughput really becomes an issue.
EDIT: As SpacemanSpiff noted below, the maximum throughput of the device can be affected by features on the device. VPN tunnels have encryption overhead and can therefore support less throughput than the device as a whole. Packet inspection (i.e. virus scanning, etc.) can also reduce the throughput. So if you had a firewall with virus scanning turned on, it might support a maximum of 40Mpbs rather than its usual 50Mbps.