Is there a limit on the number of emails that can be imported from participants, and in IRIS in general?

No, unless the file is very large, in which case it is best to split the file into several smaller files to ensure the import goes smoothly.

There are generally no limits in IRIS, whether for groups, tokens, imports, mailings, etc. However, a spring ‘clear-out’ is recommended if you do not intend to keep a long-term record of your tokens; for example, you could delete tokens that have already been used once a year.