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I have two student loans. The biggest one was consolidated when rates were at 3.25%. My graduate loan is at a higher rate. I pay them both to the same agency, on the same date. It doesn't always pay to consolidate. You need to check the interest rate for each loan.
Number of installment loans doesn't really matter as much as the amount in general. One for $50K or 19 for $50K. Although if all 19 are reporting to your reports that will significantly help your AAoA in the long run.