Whenever you draw a card, put a +1/+1 counter on Chasm Skulker. When Chasm Skulker dies, create X 1/1 blue Squid creature tokens with islandwalk, where X is the number of +1/+1 counters on Chasm Skulker.
If you draw multiple cards, the first ability will trigger that many times. Each of these abilities will cause a +1/+1 counter to be put on Chasm Skulker.
If enough -1/-1 counters are put on Chasm Skulker at the same time to make its toughness 0 or less, the number of +1/+1 counters on it before it got any -1/-1 counters will be used to determine how many Squid tokens you get. For example, if there are two +1/+1 counters on Chasm Skulker and it gets three -1/-1 counters, you'll get two Squid tokens.