Salesforce Integration User

Using a single integration user is certainly possible however you need to keep a couple of things in mind

• Data visibility
• Permissions
• tracking

Data visibility: every application can also see the data of the other applications as it all comes from the same user.

Permissions: Applications will most likely have broader permissions than they need as the permissions will be based on the need of all applications and not just 1 application.

Tracking: It will be harder, maybe even impossible to see the origin of a change if multiple applications can create and update accounts for example.

For these reasons it is best practice to use multiple users for external applications. On your other question: You can multiple API requests at the same time with a single user. just be aware that you are still bound to the concurrent API request limit on requests over 20 seconds just like when you work with multiple users.