What CRF or settings I should choose for h265 in order to achieve a similiar quality of h264?
The CRF scales for x264 and x265 do not correspond. x265 CRF 28 is supposed to be equivalent to x264 CRF 23. But x265 is not yet as mature in its development as x264, so take that CRF equivalence with a pinch of salt. 

That said, you can try to establish your own calibration between the current versions of the encoding libraries in your ffmpeg by running the following command, which executes two popular video quality metrics:

ffmpeg -i encoded-video.mp4 -i reference-video.mp4 -lavfi "ssim;[0:v][1:v]psnr" -f null -


The final lines of the console output will contain:

[Parsed_ssim_0 @ 000000000039ad80] SSIM Y:0.984483 U:0.980458 V:0.980921 All:0.983219 (17.751712)
[Parsed_psnr_1 @ 0000000000398320] PSNR y:42.63 u:43.19 v:44.09 average:42.90 min:42.07 max:46.16


So, run the command once with the x264 output and once with x265 and compare with different x265 outputs, till you get similar measures. Of course, these metrics aren't perfect but you can use them as a rough guide to establish equivalence.