Others have asked this question, and I think the general consensus has been a qualified 'no, there is no limit'. However, the truth is probably that nobody really knows for sure (except, of course, for the people at Firaxis).
Good, because my mod in the end will probably be near to 200 units. Hopefully there is not limit so I can add even more than that! <- They need to add those two emoticons together to have a crazy laugh