I won't claim it is "unlimited" but it is unlimited in Dashboard, there is no maximum stated number of rules, and no limit at which it would be considered unsupported. That said, you will come to a point where it becomes too cumbersome to examine and maintain THAT many rules on a single page in Meraki Dashboard.
For some administrators that's in the low hundreds of rules, for others it's in the high hundreds, and we do have some customers with well over 1,000 rules on a single MX. I would usually make the argument that the MX and Dashboard can handle as many rules as you might want and STILL consider it practical and be able to administer it properly. I would further argue that (unless it's a corner case) that if you needed 500, 800 or 1200 firewall rules, then it's not designed properly and should be simplified.
Same in general for content filtering, there is no hard limit to how many filters you can turn on.