I am 999% sure that no Japanese would answer like the above.
But...
san-in-iti (you mean, three, two, one, right?) or sanbyaku nijyu ichi (three hundreds twenty one) is just a personal choice.
The best professional solution should be to remember all numbers (at least the number of rooms your hotel has) in Japanese.
The 2nd best solution would be to kindly ask the customer to write the room number.
Ok, suppose the clients walks away from me (she/he has no idea a sign is required on the bill) how can I tell them something like: "please, i need a sign on the bill!"
Ok, suppose the clients walks away from me (she/he has no idea a sign is required on the bill) how can I tell them something like: "please, i need a sign on the bill!"
Okyaku-sama! Sain wo onegaishimasu.
(Sir!/Madam! Your signature please!)
When you want to address a client but do not know/remember his/her name, "okyaku-sama" is a very useful word as you can use it for both male and female.