That's not a requirement or an appropriate prescribing practice. The point in the process where the determination of the number of tablets to take and communication of that to the patient occurs in either the administration or dispensing, where it's determined by the stock on hand or preferred form. If the prescriber for some reason has a specific preference as to the tablet size, it should be written as "acetaminophen PO 1000mg q6hrs. Dispense as 500mg tablets". An order or prescription written as "500mg take 2 tablets" does not include the correct dose of 1000mg anywhere in the order, and instead includes an incorrect dosage distractor, which is a poorly written order.
Where I see this most commonly is where a prescriber thinks they're being helpful by writing a script for 25mg metoprolol as 50mg, take one-half tab, to try and save their insurance-less patient money, not realizing they are most likely going to use the commonly available $4-for-a-month supply options, in which case the prescriber is going to fill the script as 25mg tabs, but the prescriber jumped ahead in the process and already told the patient to take a half-tab, so now they are taking half a 25mg tab. And then when the patient comes into the hospital, med recs often format the PTA orders as an initial section for dose, which would in this case be 50mg, and then somewhat obscured in the comments section it says "take one half tab", these orders will be frequently misread and the patient will have their home med ordered as 50mg while in the hospital.
There's no reason to not write the initial order with the correct dose prominently included, and no reason to jump ahead and try and do the administering or dispensing step of the process at the same time as the prescribing or ordering step.