> Do you know how many leds in the strip I can control with the IC recommended by Adafruit?
All of them, the IC does not limit how many leds you can control.
> I suppose there are current limitations
The IC is only a logic level shift to convert the 3.3 V from Raspberry Pi to 5V for LED strip. There should be amount of current through the IC should not change depending on number of LEDS. It only contains the output of the SPI bus which is binary data representing something like “set LED 5 to red”. The current that powers the strip is through different power source and these have ratings such as 5V 2A or 5V 10A. The higher the maximum amperage the more LEDs you can power. I think they said something like 10 A for 5 meters in the website, but this assumes the LEDs are full white consuming the most amount of current. If they are very dim or a lower frequency color then it would draw less current and you might be able to get away with longer strip