Robert,

You declare an instance of class MARKET_DEPTH. 

MARKET_DEPTH    depth;

You can ensure that it's available with...

if (depth.available() == false)
    return RTX_FAIL;   

You can then access the max number of levels with...

int iMaxLevels = depth.maxLevels;

Then ....

int                xBid = 0;

while(xBid < iMaxLevels )
                {
                bidTotal += depth[xBid].bidsize;
                askTotal += depth[xBid].asksize;
                xBid++;
                }

Let me know if that answers your question.