There is no limitation on the api.thingspeak.com side to updating all 8 fields at once.
I don't have a SIM900A module to test with, but I was just able to test updating all 8 fields with a Mega and WiFi-101 shield using our library available on GitHub.
This issue has come up once or twice before with other hardware configuration. In the past the problem was that the TCP buffer wasn't large enough to hold the entire outgoing message when all 8 fields were being updated.