So to reiterate:
- Users have .csvs or other files where large numbers of IMEis are listed
- They need to be able to search for these IMEIs in your system
Ideally you'd have access to analytics or user interviews that could help you define the upper limit users search. It sounds like you don't have access to either, so in the meantime we can make a few assumptions.
As you said, it seems the most likely scenario is that they'll be copying and pasting these numbers, as they are difficult to correctly input due to their length. They likely won't be checking their work, again due to length, so displaying the pasted content is mostly irrelevant - you can display "1234567890abcde, 1234567890abcde, and 498 more", which should give them enough information about their search to complete their task.
The main bottleneck will likely be your backend system, not the UI. If you paste 500 IMEIs, how fast does the system respond? If it slows at any point and effects the UX, you've found your limit. If it responds adequately for 10,000 IMEIs, there's little reason to limit it at all.