On a travel day, you can use as many trains as you like. Whether or not they require a seat reservation does not matter, as long as you have one for the trains where it is mandatory. If a seat reservation is not mandatory, then you don't have to pay anything extra.

This PASS IS your ticket-and only when a travelday has been activated, which should be rather self-evident. IF you ´forget´ to do this and controller comes along, you will be considered as ´back/fraudulent rider´ and can get fined according to rules of the country you are in at that time. As passholders certainly do have a solid reputation of trying to cheat often, they will have no mercy on you, the tipical (it seems is nearly always brits, unable to even grasp that other country=other custom) feigning ignorance will not help you-how could you otherwise even have come that far from this island?

Thank you for your replies, greatly appreciated! 
To put this into perspective I have my routes planned with 6 travel days, and 3 of these days require reserving which I am happy to do. 
what I am unsure of is whether these 3 days are inclusive in my 5 day pass when I am having to pay separately anyway? Or would these not count? That way I’d still have 5 travel days available on top of the 3 I am paying for… 
If they are included, will I need to review my route to just 5 days (to meet pass requirements) or shall I buy tickets for one additional day so my full route is covered? 
appreciate the guidance received and I’m sorry to sound dumb with this! I just want to get it right :) 

The reservation is separate from the pass, which is your ticket. So if you  buy the reservations for the 3 days you will use 3 of your travel days.

You always have the possibility to purchase a ticket with a reservation for one of the days if you want to travel for 6 days. 

If you will be travelling domestically in Germany during the summer there is the 9 EUR ticket which could be interesting to save travel days.

https://www.bahn.com/en/offers/regional/9-euro-ticket-en