1 Corinthians 14:23

'If therefore the whole church be assembled together and all speak with tongues, and there come in men unlearned or unbelieving, will they not say that you are mad?'

Then he brings a second argument, taking the worst case scenario, which confirms what has been said. Suppose there is a gathering of the whole church, and suppose an unbeliever or someone untaught comes in and finds that everyone, one by one, speaks in tongues (or even all together) and nothing else. What will his impression be? He will simply say that they are all mad. So the two arguments emphasise that those who are seeking will think that God has nothing to say to them, and those who are simply curious will write them off as mad.

The case is an improbable one. There was no way that the whole church would gather and do nothing but speak in tongues. It is exaggeration to bring out the point.

This in no way indicates that all could speak in tongues, any more than the next verse means that all could prophesy. It is a theoretical case which brings out the inadequacy of tongues as an evangelistic medium (they might well have thought that what they themselves saw as something wonderful would convince everyone else as well).