You can only have 50 active custom attributes at a time.
You can delete attibutes and add attributes, but the total that can be use is 50.
If you try to add a 51st attribute, when you try to save the attribute you will get a red banner with the message,
Maximum attribute count is reached. No more attributes can be added.
We have raised a feature request with Product Management to increase the number of custom attributes from 50 to 100.
Jira Reference: ISFR-1253 - DLP - Increase the number of allowed Custom Attributes