How to know if I can craft "X" mod onto an item?

I have been having problems trying to understand this.

How can I know if I can craft some property onto an item? I have read that rare items can have amax of 6 suffixes, but I put a ring with 5 and the crafting table says it is maxed for some, and not for others...

SOmeone care to explain or link me to a calculator or something that can help me to know if I can craft before I buy something?

Rare items can have 6 _affixes_, not "suffixes". Please learn the difference between a prefix and a suffix. Specifically, rare items can have 3 of each.

http://pathofexile.gamepedia.com/Item_Affix

Memorize that entire list and you'll be able to tell at a glance for 95% of items whether they can have more prefixes or suffixes. The only difficult thing is rarity (which has prefixes and suffixes of overlapping values), though some people have trouble with hybrid mods like damage/accuracy.