wouldn't happen.......1000 SM clones-maybe but 1000 BM clones-no
RIGHT NOW Naruto can only stay in BM for like 4 min..... so if he divvied up Kurama's chakra equally into 1000 shadow clones.....divide 4 min which is 260 sec by 1000 clones 260/1000=.24 sec.....so no there wouldn't be enough chakra and naruto couldn't sustain that