diff --git a/mknapsack/_algos.f b/mknapsack/_algos.f
index 5098d5a..1fb0f52 100644
--- a/mknapsack/_algos.f
+++ b/mknapsack/_algos.f
@@ -673,7 +673,7 @@ subroutine chmtm(n,m,p,w,c,maxn,maxm,z)
 c    ISBN: 0-471-92420-2,
 c    LC: QA267.7.M37.
 c
-      integer p(1000),w(1000),c(10),z
+      integer p(1000),w(1000),c(20),z
 
       if ( n .le. 1 ) z = - 1
       if ( n .gt. maxn ) z = - 1
@@ -8243,7 +8243,7 @@ subroutine mtm ( n, m, p, w, c, z, x, back, jck, jub )
 cf2py intent(in) n, m, p, w, c, jck, back
 cf2py intent(hide) jub
 cf2py intent(out) z, x, back
-      integer p(1000),w(1000),c(10),x(1000),back,z
+      integer p(1000),w(1000),c(20),x(1000),back,z
       integer bb(10,1000),bl(10,1001),xc(10,1000),xl(10,1000)
       integer b(1001),ubb(1000)
       integer f(10),pbl(10),q(10),v(10),s,u,ub,vb
@@ -8253,7 +8253,7 @@ subroutine mtm ( n, m, p, w, c, z, x, back, jck, jub )
       common /pub/  lx(1000),lxi(1000),lr,lri,lubi
 
       maxn = 1000
-      maxm = 10
+      maxm = 20
       z = 0
 
       if ( jck .eq. 1 ) then
diff --git a/mknapsack/_multiple.py b/mknapsack/_multiple.py
index 5f11ee9..cbb3e05 100644
--- a/mknapsack/_multiple.py
+++ b/mknapsack/_multiple.py
@@ -2,6 +2,7 @@
 
 
 import logging
+import warnings
 
 from typing import List, Optional
 
@@ -123,7 +124,8 @@ def solve_multiple_knapsack(
     if method == 'mtm':
         # These are checked Fortran side as well, but would fail at padding
         maxn = 1000
-        maxm = 10
+        maxm = 20
+        warnm = 10
         if n > maxn:
             raise ValueError(
                 f'Number of items ({n}) cannot be greater than {maxn} for '
@@ -132,6 +134,10 @@ def solve_multiple_knapsack(
             raise ValueError(
                 f'Number of knapsacks ({m}) cannot be greater than {maxm} for '
                 f'method="{method}", please try for example method="mthm"')
+        elif verbose and m > warnm:
+            warnings.warn(
+                    'Using more than 10 knapsacks may cause the problem '
+                    'to take too long! Consider using fewer knapsacks.')
 
         p = pad_array(profits, maxn)
         w = pad_array(weights, maxn)