Hi Afida,
There is no maximum limit for a corpus size, but AntConc processes data in RAM memory so there is a limit there. Also, a more important restriction is that AntConc displays ALL results in the window as there is currently no paging of the results. This means that if your search processes millions of results, the RAM memory may be taken up displaying the information on the screen. In general, most users of AntConc work with corpora of less than 10 millions words, but as I say, in theory it can handle more.
I hope that helps.
Laurence.