I would use one or more tables per form template. So, let's say you intend to create templates for a Purchase Order and for an Expense Report -- those two forms have very different purposes. You'd want to create a table that holds your Purchase Order information (possible more than one table -- a Purchase Order could very easily describe a one to many relationship, with Vendor, Date and Buyer going into one table and then all the items purchased going into another) and another table (or more) to contain your Expense Report information (purchase date, approved by, etc).
You need to think in advance about how you plan to get your data into SQL. You can either create your forms off your database, which will enable InfoPath's native submit functionality, but is also very rigid (form schema, on the whole, must match table schema, can't easily change your mind about what tables you want to submit to, etc) or you can buy or build a web service that allows you to build your forms first, then submit whereever you want. I'd highly recommend our web service, of course, if that is the approach you decide to take. A free download is available here or a video demo can be found here.