Make a plan (with timetables) and then count the days yourself.
The website doesn’t always count correctly so count the days yourself and it’ll be fine. :)
On a pass day (00:00 - 23:59) you can board as many trains as you’d like.
To add : the Eurostar to mainland Europe has a limited number of seats available to Interrail/Eurail passholders (quota). It can sell out days or even weeks in advance so book this one early. 30€ seat reservation (really expensive but there’s no other alternative)
There is a limited number of pass holder seats on the Eurostar between London and mainland Europe. On popular departures and during high season those sell out weeks, and sometimes months in advance.
The best place to see the availability of passholder seats and make reservations on the Eurostar is
https://www.b-europe.com/EN/Booking/Pass#TravelWish
If you don't get any result, press "later trains" and eventually you will find the next available connection.
If you have a mobile pass you need to generate a Pass Cover Number in order to make the reservation at b-europe. You do that here in the PCN generator:
https://community.eurail.com/news-and-announcements-39/pass-cover-number-generator-is-live-5653