There is no limit on the number of demat accounts that you can have. You can have multiple demat accounts although you must factor in the additional costs and the challenges of multiple demat accounts. The first thing is to ensure that your bank account is mapped to the DP account. This will make it much safer as any credits will go automatically only to your specified bank account. Nowadays, internet trading is the in-thing and normally your demat account, trading and bank account are linked to each other in a single seamless chain.
For regular credits to your DP account, you can either choose to sign a receipt-in slip each time or you can opt for a standing instruction wherein the credits to your demat account will come automatically without the RI slip each time. This option is part of the application form. When you sell shares you are required to sign a debit instruction slip (DIS) mentioning each share that you are selling. In online trading, you can choose to give a power of attorney (POA) to your broker which makes demat selling much simpler.