The short answer is, for forest orienteering, top ten but not top five.
In the following, WOC = World Orienteering Championships, which are held annually.
The WOC sprints have qualification races whereas the forest events, middle and long, don't. Every country can enter three runners for each of the men's and women's sprints. To manage the fields of the forest events to a reasonable size (about 80), not every country can have three entries. The strongest eight countries do have three runners each, but middle strength countries have only two runners, and everyone else just one. Strength is determined by which “division” a country is in, and this is based on past recent performance at WOC. There are promotions and relegations after each WOC. There are separate division tables for men and women.
The divisions provide an indication of a country's strength in forest orienteering compared to other countries. Currently four countries are in the top division for both men and women, and you may not be so surprised to know they are Sweden, Norway, Finland and Switzerland. These countries have won 80% of WOC medals. The other strong teams of recent years have been the French men and the Russian women. So you can think of five seats in each top division being quite firmly taken. That leaves three seats in each for GB to compete for. For 2018 GB is in the top division for women; the British women had a strong 2017 WOC, and finished seventh in the 2017 top division. The GB men were in the top division in 2017, but did not have a strong WOC, came bottom (eighth) and were thus relegated.
GB are stronger at sprint, but since there are qualification races every country can enter three runners and there is no need for division tables.
There are a few extra places for additional home country runners (if not in top division), the defending champion, and region (e.g Europe) champion.
The start order does not depend on an athlete's country, it depends on their world ranking. The higher an athlete is ranked the later they are in the startlist (with a little randomness, and a special rule for recent medal winners.)
The divisions for 2018 are:
The only non-European countries mentioned above are CAN, NZL, AUS.
Incidentally the number of runners per country for the every-4-years World Games in Poland in the summer of 2017 were also determined by WOC results. Britain got places for 2 women and 2 men, which was handy as there was a sprint relay competition as well as the sprint and middle individual races. Britain was an excellent 5th in the Sprint Relay at the World Games, held in pouring rain in the park and zoo by Centennial Hall in Wrocław, Poland.