More tile images you have in library, better mosaic you'll get.

If you don't mind to use a heavy color correction you may use about 100 or 200 Base frames.

As you should remember, Base frames are single movie frames extracted at predefined interval.

If you are going to extract on image per 10 seconds you need 1000 seconds video for 100 Base frames. 1000 seconds it is about 17 minutes. This means that if you have 17 minutes or more of video footage you have good chances for nice looking and interesting video mosaic.

From the other hand, please allow me to repeat the golden rule of thumb:
More tile images you have in library, better mosaic you'll get!

Note:
To render mosaic with default settings - Repeat = <Auto>  and Distance=<6> you had to have at least 144 different tiles in your library. If you have less - you'll get <Not enough tiles> error message and in that case you had to either add tiles into library either decrease the Distance setting.