We are currently using SQL Server 2008 Express Edition, but would like to upgrade to Standard Edition. Does it mean that we need a license with 20 seats, if we have 20 Active Directory users that are using the DB from a C# application?
If yes, does it make sense to switch from Windows Forms to Web Applications in order to decrease the amount of licenses needed?
Switching to a web app won’t change the licensing needs of your application. If you have 20 users connecting to your SQL Server then you need 20 CALs for Standard Edition as whilst you may have a single “user” connecting to the DB you’re still servicing 20 users. The MS license docs cover this in some detail.
The alternative approach for to go with per processor licenses. You obviously need to do the maths to work out which option is more cost effective for your user growth estimates.
Given that you’re starting at 20 users the per user (CAL) route will probably be the cheapest option.