TH1D* isto = new TH1D*[20];
After this array allocation, create the individual histograms with
isto[0] = new TH1D("aaa0","aaa",100,0,100);
isto[1] = new TH1D("aaa1","aaa",100,0,100); etc
or
for (int i=0;i<20;i++) {
isto[i] = new TH1D(Form("aaa%d",i),"aaa",100,0,100);
}
To delete these objects
-delete first all histograms
for (int i=0;i<20;i++) delete isto[i];
-then delete the array