I'm assuming you're looking for something a lot less expensive and less...robust...than what you've probably found online. A lot of the bigger packages/companies also will not even look at a single 20-unit property.
I have used TenantFile in the past a single-location small property and was happy with it. It doesn't have all the bells & whistles, but does pretty much everything you need it to. And it's much more affordable.
www.tenantfile.com