The is no mandatory reservation on domestic trains in Germany. But, if you will be travelling during the summer on a popular departure, it can be wise to make a reservation. If the train gets too crowded the train staff for security reasons can will ask persons without reservation to leave the train. 

On bahn.com and in the DB app there is information about how crowded DB think the train will be.

Ok thanks

And its 4,50-2nd cl-and it even incl. 1 chnage =2 trains, if done right away-do it via bahn.com ´seat only´ you only get a mail stating date/train/car/seat nr. ON the platforms are screens which will show in what order the cars are set to arrive-so you can roughly stand in right place. ICE has a dreadful report now for being late-around half the trains are cancelled or more late as 15 mins-so connections are very often missed-but you may even be able to catch a train that should have left 50 mins before you arr station.

IF you want to take the trouble and if you dont mind sitting long time on computer-you can make first yourself-for free- bahn.bonus member-get 500 free points-and 1 pt for ev. € spent-from 500 pts you can get ´presents´ like free upgrade to 1st (NOT on a pass-on a normal ticket) or other.

So, do you mean that if I select ‘seat only’ on the website of the deutsche bahn, I don’t have to pay?

For future travellers: deutsche Bahn charges a fixed reservation price per journey, regardless of how many trains that journey includes. @mcadv has been told so before but apparently keeps forgetting.