Log lines: 
k1=doesn't matter, k2=doesn't matter, k3=[v3, v4]
k1=doesn't matter, k2=doesn't matter, k3=[v5, v4, v6]
k1=doesn't matter, k2=doesn't matter, k3=[v5, v6, v9, v4]
Expected output:
v4 => 3
v5 => 2
v6 => 2
v3 => 1
v9 => 1
 
					
				
		
Hi @hpendela
Please try
| makeresults 
| eval log="k1=doesn't matter, k2=doesn't matter, k3=[v3, v4];
k1=doesn't matter, k2=doesn't matter, k3=[v5, v4, v6];
k1=doesn't matter, k2=doesn't matter, k3=[v5, v6, v9, v4]" 
| makemv delim=";" log 
| mvexpand log 
| rex field=log "k3=\[(?P<k3>(.*))]" 
| makemv delim="," k3 
| eval k3= ltrim(k3) 
| stats count by k3
 
					
				
		
@hpendela,
Try
"your base search" | rex field=k3 max_match=0 (?<vals>\w\d+)|stats count by vals
If  you have more than one character in v , change the grouping part as (?<vals>\w+\d+)
