For three people, you can say “San nin desu. ” If you are eating alone, say “Hitori desu. ”
If they don’t have one available, do your best to look up Japanese on a mobile device (if available) or work with what Japanese you know.
You put the numbers together like this: 19 is 10 + 9 so it is juu-kyuu (十九). 90 is 9 sets of 10 so it is kyuu-juu (九十). So 198 is hyaku-kyuu-juu-hachi (百九十八).
If you want more than ten of something, specify using normal numbers—juichi, juni, jusan, etc.
Don’t stick your chopsticks upright in a bowl of rice. This is only done at funerals.