Usually shop assistants in flower shops make bunches of odd number of flowers on default unless you say that you need even number. But be cautious, if they ask you about number of flowers you want an if you say f.e. ten (cause it's ok in your culture), they probably won't ask you again (to warn you about the tradition) because they may think that you buy flowers to a funeral or to visit someone's grave and this topic is quite private to discuss. So at least be smiley to let shop assistant know that you are not buying flowers by a mourning reason and make them correct the number of flowers to odd.