Of course, there is nothing stopping you from using your database for any number of sites. Note:This could cause performance issue depending on how much information you are pulling from the SQL server for each site, just something to consider since I'm not sure how many sites you plan on having use the same database.
Yes you can. There will be separate connection for each websites to your database. Number of simultaneous connections allowed can be customized in your database settings.
I think you maybe referring to the MaxPoolSize and MinPoolSize setting on the connection string. Try looking at this link. http://msdn.microsoft.com/en-us/library/8xx3tyca(v=vs.71).aspx
