I want to add thousands of new records and I want to make sure it will work.
There is no limit to how many records can be uploaded at one time. The max file size for an upload is 10mb, but this should be able to handle a large number of records. If the file is bigger than 10mb, you can split the list into sections and import each one. There may be charges for certain thresholds of records, but you will want to check with your CSM to verify.