I don't know
ack but with tools such as
grep I typically use:
( find . -maxdepth 1 -print ; find . -mindepth 2 -print ) | xargs -n 50 -exec grep TXT
The part between
) ensures that first the files at level 1 are listed and after that those at 2 and deeper (you can vary).
xargs feeds the file names per 50 to
Of course it depends on which variant of
find is available on your platform. If running something from 30 years ago, you will need to use something like sorting on the number of forward slashes.