Welcome to the board
The best way is to use a fabric material like PVC and either cut the numbers individually, or cut them out reversed from the fabric and glue them on. There should be companies that will do the reverse by waterjet or laser. My friend and I printed numbers out, he hand cut them with scissors, then we glued them onto the boat with a two part glue.
If like here in the USA you also have registration stickers (decals) they too will need to be put onto a stiffer surface that won't expand or contract. They can be made with thin but harder plastic that conforms to the hull. A surround glued to it makes it pretty.
https://www.boatnumberplate.com/inflatable_boats
These are my numbers for an example. I downloaded the letters and numbers from a website, blew them up to the desirable size per regulation. I used a private driveway sign cut up for the backer to the decals. Already had thin grey PVC in stock that cuts easily with sharp scissors, along with Stabond 2 part glue.