Correct ... one mediator. Only Jesus will be the one to stand as our mediator between us and God on our last day. That is something totally different than asking one of God's saints for prayers on our behalf. That's not mediation.
Main Entry: me·di·a·tor
Pronunciation: 'mE-dE-"A-t&r
Function: noun
: one that works to effect reconciliation, settlement, or compromise between parties at variance
If I had a nickel for all the Catholic websites and Catholic comments that say Mary helps in reconciling men to God I'd be rich.