Hello Marcia and welcome to the forums.
20 SMB connections it is; Type winver and scroll
From
Microsoft Software License Terms, Section 3, subparagraph f.
Device Connections. You may allow up to 20 other devices to access software installed on the licensed computer to use only File Services, Print Services, Internet Information Services and Internet Connection Sharing and Telephony Services.
Which can be additionally confirmed by opening a command prompt as administrator and typing
net config server
and hit enter
look at the "Maximum Logged On Users" line, should also say 20.
Hope that helps.
Additionally you'll probably want a bit of beef in the hardware specs for the machine (processor speed, memory type and amount, as well as hard drive performance, consider SSDs or 10,000 rpm SATAII.
Additionally some users have reported issues with Win7 hosting shares used by some downlevel clients like XP. Take a look at this article for further information and consideration
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and this one to help get you started from microsoft
Networking home computers running different versions of Windows
Finally mirroring your hard drives (Raid 1) is a good idea, but is no substitute for a comprehensive backup scheme combining system images with critical data backups.