Mower numbers on a walk behind are always on a sticker or plate on the rear of the deck.
Riders tend to be trickier, the numbers are either under the seat or somewhere on the body.
Riders sometimes have deck numbers, so look for those too. Try to give us as much information as possible about your problem, this will prevent us having to collect more information from you to solve your problem, thus delaying your solution. Any other information you can provide that me be prudent will help us greatly.