There were 3 lions.
Because of formatting it is hard to show how I came up with the answer.
I made a grid showing location and put in the known information:
bushes - 2, ?
grass - ? zebra
trees - +4, (not chimps/antelopes)
boulder - (not chimps/antelopes)
waterhole - ?, ?
Then I wrote this sequence based on the clues:
(antelope/zebra) < lion < chimp < giraffe < (antelope/zebra)
Then we know that the group of two in the bushes can only be antelopes.
Then we know that the group in the trees can only be giraffes.
That leaves the lions at the boulder and the chimps at the waterhole.
Then I just plugged in the numbers according to the sequence.
This is what the finished grid said:
bushes, 2, antelope
grassland, 6, zebra
trees, 5, giraffe
boulder, 3, lions
water, 4, chimp
Let me know if I'm right!