Sabaody there was one celestial dragon and pirates are supernovas while on gold valley there are the most or everyone and pirates are rocks and roger.
Kong and Garp knew the possibility than they could attack them. So there are any escuse than at least two admirlas where on gold valley
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but all the strongest pirates on that time (garp and tsuru) are viceadmirals
The Celestial Dragons didn't have a lack of fighting force on God's Valley. The God's Knights were there, Saturn was there, Garp was sent to help, and they had gathered a plethora of strong Celestial Dragons to compete in the competition. If Saturn thought they needed more help ,especially an Admiral or two, he would have personally requested it.
The Navy acts on the Gorosei's orders, and a member of the Gorosei was on God's Valley, and didn't think it necessary to order an Admiral to come.
Now obviously the situation ended up much more dire than expected, but at that point, an Admiral wouldn't have gotten there in time.