The solution is perfectly explained here:
https://stackoverflow.com/questions/39589696/ceph-too-many-pgs-per-osd-all-you-need-to-know
To keep the solution safe, I'll write here down it.

ceph pg dump | awk '
BEGIN { IGNORECASE = 1 }
 /^PG_STAT/ { col=1; while($col!="UP") {col++}; col++ }
 /^[0-9a-f]+\.[0-9a-f]+/ { match($0,/^[0-9a-f]+/); pool=substr($0, RSTART, RLENGTH); poollist[pool]=0;
 up=$col; i=0; RSTART=0; RLENGTH=0; delete osds; while(match(up,/[0-9]+/)>0) { osds[++i]=substr(up,RSTART,RLENGTH); up = substr(up, RSTART+RLENGTH) }
 for(i in osds) {array[osds[i],pool]++; osdlist[osds[i]];}
}
END {
 printf("\n");
 printf("pool :\t"); for (i in poollist) printf("%s\t",i); printf("| SUM \n");
 for (i in poollist) printf("--------"); printf("----------------\n");
 for (i in osdlist) { printf("osd.%i\t", i); sum=0;
   for (j in poollist) { printf("%i\t", array[i,j]); sum+=array[i,j]; sumpool[j]+=array[i,j] }; printf("| %i\n",sum) }
 for (i in poollist) printf("--------"); printf("----------------\n");
 printf("SUM :\t"); for (i in poollist) printf("%s\t",sumpool[i]); printf("|\n");
}'

That's all

Procedure for dropping pool: [HOWTO] Completely remove a POOL from cluster Don't drop the pool, continue reading.

If you're sure that you need more PG's per os, change the configuration.
You must add:

mon pg warn max per osd = ${NEW_PG_PER_OSD_NUMBER}

on your ceph.conf under the [global] section.

And update the config worldwide.

You can also change the setting without having to restart:

# check
ceph tell 'osd.*' config get mon_max_pg_per_osd
# change
ceph tell 'osd.*' config set mon_max_pg_per_osd ${NEW_PG_PER_OSD_NUMBER}
# check again
ceph tell 'osd.*' config get mon_max_pg_per_osd