Microsoft Exchange server, including Office 365 Exchange Online and Outlook.com, as well as on-prem Exchange servers, limit the number of rules a user can have in an Exchange mailbox. The limit is based on total size, not number of rules, so one user might have 20 rules while another has 30.
Exchange 2003 and older were limited to just 32 KB worth of rules. (Yikes!) The limit was increased beginning with Exchange Server 2007, to 64KB by default and configurable up to 256KB. Later versions of Exchange dropped the lower default setting and gave everyone the maximum 256KB. This includes Office 365 Exchange and Outlook.com accounts. (Administrators can lower it.)
This increased storage space allows in the neighborhood of 100 rules, or about 90 more than I want to manage in the Rules dialog.
Really old versions of Outlook stored client rules in the client and server-based rules used the limited storage space, allowing users to have many more rules. This little trick no longer works as all rules, both client and server, are stored on the Exchange server and subject to the size limit.
Manage the Rules Quota
If you mailbox rules quota is less than 256KB, your Exchange administrator can increase the limit, up to the maximum of 256KB, using the following cmdlet in the Exchange management shell to raise the limit to 256KB. See Use PowerShell to log into Office 365 Exchange if you need the complete steps to log in.
Set-Mailbox username -RulesQuota:256KB
To check the current limit for a user, open Exchange Management Shell and run
get-mailbox username | select name,rulesquota
If users still hit the rules limit, they need to review their rules to see if any can be combined or shortened. The use of search folders or custom views may be able to reduce the number of rules in use.
See Set-Mailbox cmdlet (TechNet) for more parameters to use with the Set-Mailbox cmdlet.
See the tools listed below to overcome this limitation and for better rules handling. For a VBA solution, see VB Email Filters (Outlook Forums)