Should I set max pool size in database connection string? What happens if I don’t?

Currently your application support 100 connections in pool. Here is what conn string will look like if you want to increase it to 200:

public static string srConnectionString = 
                "server=localhost;database=mydb;uid=sa;pwd=mypw;Max Pool Size=200;";

You can investigate how many connections with database your application use, by executing sp_who procedure in your database. In most cases default connection pool size will be enough.