Industrial glue........stuff
SIKAFLEX is just a brand name. Some boat/yacht builder supplier have them in their catalogs, some hardware store sell it as well (e.g. Bauhaus in germany), car tuning or body shops need this stuff too.
You do not have to use SIKAFLEX. You can use any comparable adhesive, car body stuff, boating adhesive to glue windows in, industrial stickystuff...........a lot of brands produce stuff for the same purpose.
Google the technical data sheet from SIKAFLEX, then go on and google the tech-sheets of the stuff you can buy close by and/or cheaper. Go and buy close by.
How many?
1 tube is definitely too less.
2 might work out
3 is definitely enough and piece of mind
4 is like 3 but you can practise.........(i think working with that stuff is comparable to working with pro seal)
5 is like 4, and you will end up walking through your house (car) looking for things to be fixed/glued......