Risk register and environmental aspects

1.can we make a single risk register for a company as a whole or should we have to make functional/ departmental wise or the option left to the Organ.?

The standard does not define how organization will compile its risks register, so you can do it in any way that you find the most suitable. For more information, see: How to address risks and opportunities in ISO 9001 https://advisera.com/9001academy/blog/2016/06/21/how-to-address-risks-and-opportunities-in-iso-9001/

Regarding EMS 2015 standard
1.Should we have to make risk register for EMS also as per cl.6.1

Yes, you should make risk register for risks and opportunities related to the EMS and you can do it in the same way as for ISO 9001. For more information, see: Risks and opportunities in ISO 14001:2015 – What they are and why they are important https://advisera.com/14001academy/blog/2016/03/07/risks-and-opportunities-in-iso-140012015-what-they-are-and-why-they-are-important/

2.For Aspect Impact study is their any template / format to use?

Yes, we do have templates for identification and evaluation of environmental aspects and impacts. Here you can download free preview of our Procedure for Identification and Evaluation of Environmental Aspects and Risks https://advisera.com/14001academy/documentation/procedure-for-identification-and-evaluation-of-environmental-aspects/