If we have 7,000 Sales Reps, each sales rep creates 2,000 records per day. So the total records in a month will be 7000*2000*25 = 350,000,000. It’s a large data, how the salesforce handle these data?
Technically this is no problem. You might only need to buy extra data volume. The following should help you to estimate your required and available volume.
You schould be more specific about the Objects or Entities you are dealing with. E.g. Opportunity, OpportunityLineItem, OpptunityContactRole, OpportunityLineItemSchedule, Product, Activity or CustomObject? Memory will be counted different for some of them.
Let's assume CustomObject (which is also the highest consumption), each record will require 2k space.
The default available space will be calculated depending on the number of your users and their license type. Here you can see how this is calculated with examples
If you need more than default, you should talk to salesforce because extra storage pricing might depend on your special use case and is in my experience highly negotiable. E.g. some of our clients are using marketing automation tools which are creating bizillios of records of tracking data. Salesforce was very flexible in those cases.
Also if you reach 100% my experience is that they won't pull your plug immediately. Usually you have time to talk with them while your users can still work and add even more records. In other words the storage limit will be monitored but typically not enforced in real time.
So one part of loading a large volume of data will be the licensing requirements for storage. I think Uwe has largely covered that off.
The other consideration will be how you work with that data. For example, you will need to follow best practices for the bulkification of triggers.