I thought this was an interesting question, but it makes sense to have some concern about it. If you are using Sequences over identity inserts, this typically means you need more control over how those numbers are handled from creation to archive. So what if you need a lot of different Sequences stored on your server to handle your needs? Will you run out of Sequence objects that you can create?
This information is not intuitively simple to find, this is one of those times where you need to read various articles and connect the dots. Let’s start simple:
What is a Sequence?
A sequence is a user-defined schema bound object that generates a sequence of numeric values according to the specification with which the sequence was created.
MSDN on Sequence definition reference.
Ok, it’s a schema bound user defined object. What does SQL say about how many User Defined Objects we can have?
Database objects include objects such as tables, views, stored procedures, user-defined functions, triggers, rules, defaults, and constraints. The sum of the number of all objects in a database cannot exceed 2,147,483,647.
MSDN on maximum UDF reference.
Summary answer: You can have 2,147,483,647 objects in a database. One of those objects referenced would be Sequence Objects which Sequences fall into that category. This is not explicitly stated, but it can be contrived from the wording “… the sum of the number of all objects in a database cannot exceed 2,147,483,647.”