When you stand on an Arcane Circle, the strength of the Arcane Focus equals the number of spellweavers standing on the circle (all having to be +/- 10 skill points within the caster's spellweaving skill).
When you're standing on a Pentagram, the strength will be 1 level lower than the number of spellweavers.
You cannot cast a Focus with yourself, i.e. on an Arcane Circle a Focus with the strength of 1 is not possible.
The maximum strength of an Arcane Focus is 5. However, when you go to the Arcane Circle in the Prism of Light, the strength of the Arcane Focus can reach 6. You will get a +1 bonus on your Focus when casting there.