The first limiting factor is most likely going to be your license, which is limited to a specific number of gateways.
Even if you have a license for more than, say, 25 gateways, you’re probably going to want separate log servers and/or possibly multi-domain management for scalability reasons.
Also, pushing policy to more than a handful of gateways at once is going to take a while unless you’re using SmartProvisioning/SmartLSM, which has an entirely different policy deployment model.
Accelerated Policy install with R81 gateways/management will help but it only works for certain types of policy changes.
Seriously, though, if you’re going to manage a large number of gateways, it’s a good idea to engage with your local office to validate the design.