If i get 2 x Sonos Play:5 speakers and only want to stream music from a phone or a computer, do I need any additional equipment like an amp?
No, the Play:5 has a Line-In port.
https://support.sonos.com/s/article/1080?language=en
But you don't need one to set up a music share or play music that's stored on a mobile device.
»And the world is like an apple whirling silently in space, Like the circles that you find in the windmills of your mind.« (›Windmills Of Your Mind‹ [1967]. Music by Michel Legrand ; English lyrics written by Alan & Marilyn Bergman)