Thirteen roses: the number, and who actually notices it
Let us be straightforward about this count. Some recipients react to the number thirteen, and they react before they look at the flowers. Others have no feeling about it whatsoever and would never think to count the stems in a bouquet they were given.
Which of the two you are dealing with is not something you can guess from the outside.
Counting is a habit, not a rule
People who grew up where the parity of a bouquet carries meaning count without deciding to. The number registers in the first minute, along with the colour and the wrapping. People who grew up without that habit see an armful and nothing more.
Thirteen is an odd number, so in traditions that reserve odd counts for the living it is technically correct. The hesitation attaches to the figure itself rather than to parity, and it travels through stories and buildings and seating plans rather than through flower shops.
A fair number of orders go the other way, and this gets forgotten. Some customers choose thirteen deliberately, because it is a date, an anniversary, a house number or simply their own lucky figure, and for them twelve or fourteen would be the mistake.
What to do when you are not sure
The simplest approach is not to make the count a subject. It goes unwritten on the card and unsaid at the door, and in most deliveries nobody verifies it.
The second approach is structural. Set the same count in a hat box or a foam arrangement and the heads sit shoulder to shoulder in a closed circle. Counting them from above then takes deliberate effort. A hand tied bouquet in paper does the opposite, because the stems gather at one visible point and the eye follows them. That is not a trick so much as a property of the format, and it works whichever number is inside.
The third is to change the number and stop worrying. Fifteen looks almost identical in the hand, since the diameter barely moves over those two stems, and it raises no questions at all.
The fourth applies when the figure is the point. If thirteen stands for a date or something shared between two people, one line on the card turns it from a superstition into a message, and the whole issue disappears.
What the bouquet is like
Thirteen heads close into a dense, properly round top that still travels in one hand. This is the last count in the range that a person can carry from the car to the door without changing grip.
The vase wants a neck about the width of a palm. Squeeze that many stems into a narrow one and the dome stretches into a column, with several heads pinned by their neighbours and opening unevenly on one side.
Ask for calibrated heads at this size. Thirteen blooms of the same grade hold the circle, whereas a mixed batch leaves the outline lumpy, and on a closed round shape that shows from several paces away.
If you know the recipient in Lima takes omens seriously, take a different count and spend the difference on stem length. And if you know they do not, thirteen roses are simply a good, full bouquet with nothing else attached to them.