When should you think about scaling things out sideways versus scaling them up?

hey everyone, i've been thinking a lot about scaling strategies lately. we've mostly been doing vertical scaling, just beefing up our current servers. but i'm wondering when it's the right time to start thinking about horizontal scaling, you know? what are the key indicators or pain points that tell you it's time to add more machines instead of just a bigger machine? i'm curious to hear your experiences and what made you switch