If a CR 10 is an encounter for a party of 4 level 10 characters. How do I balance the encounter for 2 level 10 char or even 6 lvl 10 char? What is the new CR? (I can't seem to figure out this information from the DMG). Thanks.
You can use this encounter calculator.
The EL of an encounter increases by 2 with each doubling of the number of monsters present (and thus decreases by 2 for each halving of the number of monsters). Increasing/decreasing the number by 50% warrants a +1/-1 to the EL of the encounter.
A party of 2 level 10 characters should thus be able to handle an EL 8 encounter (or a CR 8 individual). Likewise, a party of 6 level 10 characters should be able to handle an EL 11 encounter (or a CR 11 individual).