hi
how i can disable 802.11 B on r52hn?
I want only 802.11N and G

what is the reason for that as that does not impact other users on RouterOS access-point.

But to disallow B - you have to unselect basic and supported B rates. Just be careful do not dig into a/g or n rates.

On specific commands and settings see wiki.mikrotik.com (manual)

when my r52 mode is B/G/N , the rate of my client at registration tab is 1mbps and it takes about 20-30 minute to rate and ccq goes up ( long time)

select advanced rate selection. That will speed up higher rate selection. Also, if no traffic is passign thorugh the link CCQ can be low value due to low packet count at all.

you mean that rate and ccq is depending on traffic passing to client? and this long time is usual?

i do this:
1- set mode to Only G
2-set rate to configured
3- select only rate from 24 to 54 and disable B rates

by this the clients at first seconds of connection have rate 24 to 54 and high ccq and troghput.
is this ok?

with initial signal not that high you might have problems connecting users to your AP. Set rate-selection to advanced. that will solve the issue. because disabling lower data rates is dangerous.