من السعودية XD Took me a moment to understand that.
This is true, but as far as I know if the first word, which can kinda be referred to as the subject of the senctence, is جمع مؤنث سالم (plural created by adding ات to the word) then the second word could be either singular or plural correct. Example:هذه سيارات كثيرة (These are a lot of cars)
هذه سيارات كثيرات (Also "These are a lot of cars)
Here the word كثير/كثيرات both can be used, but هذه (this) cannot be replaced with هؤلاء (these) because هؤلاء can only be used for humans.
Both are correct. I'm not really sure, but it was in my sixth grade grammar book XD