Flowers have an etiquette of their own that suggests some general rules to consider every time we give a flower or a bunch of flowers to someone.
Here’s some interesting suggestions that may help you to avoid mistakes.

The right kind of flower

In some occasions flowers play a very important roll. In these cases the choice of the right kind of flower falls on the orchid. In this case it is better to choose small ones disposed along a long branch. Even if they do not last as long as the other specie (with a big flower), they are more elegant and delicate.
When offering a single orchid it should be presented in an elegant gift pack.

A big bunch or a graceful bouquet?

Giving huge bunches of flowers, especially very bulky ones, is not of good taste. High wood steles of roses beyond a meter will be useless, it is better to choose a small bouquet, manufactured with great cure maybe with some mignon roses. Some exception can be made with the loved person, to which also a single flower will be always appreciated.

Flowers or green plants?

Not everybody loves flowers and often, if we do not know well enough the receiver, it is better to choose a beautiful apartment or balcony plant instead.
For a professional or a person of care that we would like to thank, a classic green plant is always more opportune.
Also, be very careful when giving flowers to a sick person: in such circumstance absolutely avoid flowers with great perfumes; prefer, without doubt, completely odorless species or a green plant..

The importance of color...

A great attention should be put in the choice of the color since to this it is attributed a certain symbolism.
The red colors, in particular if of a rose, have a very precise meaning, that on of burning love.
Attention also when giving a yellow flower to the fiancé since it is a symbol of betray.
The white color usually is reserved for the spouses, or for recurrences such as baptisms, communions, but however it is always a very refined color.
For Christmas the traditions wants red to be the chosen color: everything that can be found with that color brings joy and happiness.
Finally, there is no particular problem for the other colors ( rose, blue, light blue, yellow, orange and lilac ) they also have precise meanings, but the kind that generally don’t have contraindications.

... speaking of the number of the flowers: better uneven

Like already pointed out, a single flower is generally indicated only for the loved person as a token of love. In this case whichever specie will be appreciated, also one simple violet picked by the side of a road.
In most cases, when someone gives large bunches of flowers or a bouquet, who receives them probably will not count the number of the flowers; but in some particular occasions ( anniversary, engagement, etc), few but rare and valuable can be enough.
If a bunch of flowers of a single specie is offered it would be however better that the number of the flowers was uneven, above all if the person who receives them is supesticious.

Flowers, also for men

Flowers can be also given to men, but it should be avoided fragile flowers that have romantic meaning or a tenuous colors. The colors must be pronounced, (red, yellow or orange ) as it should be very decisive the design of the corolla ( red tulips).
Roses, but of lively color, will also go well. Also the package should be appropriated, avoiding silk or velvet ribbons, but preferring an opaque cotton ribbon, possibly that matches the dominant color of the flowers.
For the doctors ad other professionals, in any case, a green plant always represents a very appreciated homage that can be placed on the waiting room, better if the chosen specie doesn’t demand particular cures.