In the past the maximum number of transport rules that you could create in Exchange Online (Enterprise, Education, and Government plans) was 100. This limit has recently been increased to 300 (for Enterprise, Education, and Government plans)! For those that were concerned about reaching the 100 rule limit, you now have 200 more rules that can be created. This increase has been updated and noted on the following TechNet page.
Exchange Online Limits – See the section Transport and Inbox rule limits across Office 365 Options.
When thinking about manageability and troubleshooting, I would not recommend creating anywhere near 300 rules. I actually wouldn’t recommend creating close to 100 rules. In certain cases though, especially with tenants that have a large number of domains, a large number of rules may be needed. For those that are in that boat, this change is for you.
On a side note, I think this is the shortest post that I have written to date.