Validating Multiselect dropdown

Hi!
How can I limit (validating) the selected options of a Multiselect dropdown? (for example three)

Thank you healyhatman, but it is not working...
I also tried with "Validate data" action (relative_topics/maxSelected[3]:errormessage) and also no success.

OK got it for the front end.
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maxCount[#] and minCount[#]
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Still haven't figured out the back end sorry. For now you can just use custom PHP validation. Something along the lines of
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if(count($this->data("dropdown_field")) > 3) { return false; }

It can be done with the validate data action, but it will require you to make changes to Chronoforms code. I can provide those changes for you, but they'll be overwritten if you update CF or CC. Better off waiting for Max to fix it in a later update, and just validate with PHP for the backend.

maxCount[#] and minCount[#] is yhe solution!
Thank you healyhatman