We have different types of typology rules such as Filtering, Fatigue and Control rules
In Control rules we can able to check for delivery size, like, 'To deliver target count' is less than 50k or not, and you can make warning or Error alert if the record is greater than 50K. So if incoming target is 60K, this rule will give you warning or Error Alert (as per configuration), But it won't be limiting the count to 50k and send delivery to 50K alone.
To Achieve this, As mentioned by craigthonis, you can set up a deduplication activity and deduplicate the target based on Email, and followed by a segmentation activity.
In Segmentation activity, go to Limitation tab > select type of limit as random or ordered sampling based on your requirement > Now specify the size Limit of the segment, Maximum size equal to 50K