I have limited memory in my laptop, can I practice all ltm labs using 1 ltm device only.? please suggest 

8 GB of RAM i5 laptop is enough for building F5 ltm lab with 2 ltm devices, 1 switch, 1 routers, 2 linux servers and 1 windows machine (laptop can also used). Better configuration is always good. You can also look out for ltm virtual lab from uninets. 

If you are just looking for ltm lab, then your laptop configuration is fine. However if in future you are planning to learn more technologies like F5 DNS, ASM, APM etc. then you have these options

- Build your own server (physical server)

OR 

-Take a server on rent from uninets which will be having pre-installed eve-ng and other required images

OR

-Just purchase specific virtual labs to learn technology