How Can I Increase the Limit on the Number of Assets in Review on Adobe?

When you've earned some sales and approvals, your limit will likely increase.  Not before then.

Focus less on quantity and more on quality. 

No one in the user community knows what criteria Adobe uses to decide when to increase the limit. It is widely understood that gaining some sales history in your account is one of the factors, and also possibly your acceptance rate.

No. There is no way to increase the limit. It may depend on the number of assets sold before the limit will be increased. Rejected images affect the limit in the sense that a rejected asset will allow you to replace it with another assets, in the say way that accepted assets will allow you to add more assets up to your 501 limit.