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You can't reliably use LTA, RTA, or ETA fields (Long, Rich, Encrypted) in formulas, but you can in Apex Code. My solution would be simply:

trigger CheckWorkCount on SObject (before insert, before update) {
    for(SObject record: Trigger.new) {
        Integer count;
        if(record.RTA__c != null && (count = record.RTA__c
            .normalizeSpace() // Don't count duplicate spaces
            .stripHtmlTags() // Remove all HTML
            .replaceAll('\\S','') // Remove all non-spaces
            .length()) > 99) { // More than 99 spaces is more than 100 words
            record.RTA__c.addError('Please limit your input to 100 words or less. You currently have '+count+' words.');
        }
    }
}

I don't know of a non-code way of doing this, unfortunately.

If you want to store the count in another field, you can do that as well.