you can remove up to 4 in a straight line in one course then leave 3 and remove another 4 etc
if you get a collapse it is limited to 3 in the course above, 2 in the course above that in a triangle, which is manageable for repair and wouldnt be catastrophic to structure.
take 2 and leave 2 is fine and will probably be enough to clear the cavity
The other thing to remember is when rebuilding, you can never push enough compo in the top gap to really be happy, so I always pack some small stones in the top gap before pointing it
you can remove up to 4 in a straight line in one course then leave 3 and remove another 4 etc
if you get a collapse it is limited to 3 in the course above, 2 in the course above that in a triangle, which is manageable for repair and wouldnt be catastrophic to structure.
take 2 and leave 2 is fine and will probably be enough to clear the cavity
The other thing to remember is when rebuilding, you can never push enough compo in the top gap to really be happy, so I always pack some small stones in the top gap before pointing it