Planner buffer question

[e-zeki]

Hello, I'm fairly new in GRBL world. I've made a 3 Axis machine for myself with this build.
I've implement a streaming GCode routine in order to fill both RX buffer and Planner buffer efficiently and make it run smooth, it works great most of the cases.
I look how many bytes are available in RX buffer than I look Planner buffer how many moves I can send then I both substract line bytes and 1 movement from planner buffer as well. But I run into some faulty movements especially after an arc or a curved carve moves. I realized while linear Gcode lines are consuming 1 planner buffer spot, Arc movements or circle lines consumes more than one spot when I stream.
Is it random how much spot a line can consume in planner buffer or is there a consistent number here?
Any clarificatioun would be great
Best regards
Zeki