Learn Portuguese (Portugal): Quantas mentiras ele contou!

In English, you "tell" a lie, but Portuguese learners sometimes try to translate this using dizer (to say/tell). In Portuguese, the natural set phrase is contar uma mentira (literally "to count a lie" or "to tell a tale"). You use contar for telling stories, jokes, and lies.

Is the word order the same if I want to ask this as a question?

Yes! In Portuguese, exclamations and questions usually have the exact same word order. If you change the exclamation mark to a question mark (Quantas mentiras ele contou?), it becomes the question "How many lies did he tell?". Your spoken intonation is what makes the difference clear to the listener.

Why does Quantas end in "-as" in this sentence?

The word quanto (how much/how many) changes to match the gender and number of the noun it describes. Because mentiras (lies) is a feminine plural noun, quanto must become the feminine plural Quantas.

Could I use the verb mentir (to lie) instead?

You can use the verb mentir, but you would have to change the structure. You could exclaim O quanto ele mentiu! (How much he lied!). However, because this specific sentence focuses on counting the individual lies ("How many lies"), using the noun mentiras with the verb contar is the most natural choice.