While there is no limit on the number of simultaneous connections by a single login, other than the usual @@MAX_CONNECTIONS value, using one login for multiple users or developers is generally frowned on because doing so makes it difficult or impossible to limit permissions on a per-person basis. (You may still be able to identify/contain people based on something like a workstation name, but this would require much hackery on the DBA's part, while simply using seperete logins addresses the issue.) Such "well-known" logins also have a way of getting hard-coded into things, along with the well-known password, and then developers and users become very resistant to ever changing that password. With so many ways to get onto a corporate network these days (a rogue LogMeIn running somewhere, for example) being able to turn database access off via an AD group is beneficial.
Using a single login for a web application, which can have many simultaneous users, is more common. It might not be feasible to give out SQL Server logins or AD logins to individuals for large sites (for example, facebook). In that case, there is a benefit in that IIS can use connection pooling.
I haven't worked in an environment where it was standard operating procedure to use a shared login like that for many years, although there were some smaller/older projects where a shared login was used (and sometimes abused).