I am creating a C# billing software. The client requirement is around 30,000 entries per month. Which DB should I use if I have to integrate a mobile application with this software?

 For a billing software that needs to handle around 30,000 entries per month and also needs to be integrated with a mobile application, you have a few options for databases to use. Here are three possibilities:

1. SQL Server: If you're already using Microsoft technologies, then SQL Server may be a good choice. It's a relational database management system that's been around for a long time, and it can handle large amounts of data. It also has good integration with .NET and other Microsoft tools. SQL Server has a mobile edition called SQL Server Compact Edition that can be used on mobile devices.

2. MySQL: MySQL is another popular choice for databases. It's a free and open-source relational database management system that's widely used, and it can handle large amounts of data. It's also compatible with many programming languages, including C#. There are also several mobile versions of MySQL available that can be used on mobile devices.

3. MongoDB: If you're interested in a non-relational database, MongoDB is a good option. It's a document-oriented database that's designed for scalability and flexibility. It can handle large amounts of data and is good for handling unstructured data. MongoDB also has a mobile version called Realm that can be used on mobile devices.

Ultimately, the choice of database will depend on your specific requirements, the features you need, and the skills of your team. Consider factors such as scalability, data integrity, security, ease of integration with other technologies, and the size of your team when making your decision.