Hello,
for 5V, there is no specific limit, as you draw directly from USB Type-C connector. You should just ensure the power drawn by external devices connected on the 4 x USB Type-A (up to 500mA each) added with the 5V power drawn on your expansion is lower than 5V 2A (this give a comfortable 5V 1A for the remaining of the board).
For 3.3V, supplied by STPMIC1A Buck4 rated 2A, I did not have split of power used inside the DK2, but at a first guess, up to 500mA for expansion should be ok.
Regards
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