Hi [NAME],   Unfortunately, maximum unit for VLT domain is 2 therefore you will not be able to add more switch. 
Hi, In my environment, we have 2x Spine switches peered with VLT, however it is running out of ports. Is there anyway we can add switches to the VLT domain just like stacked switches? Will mVLT (Multi-domain VLT) help in this case? Thank you.
Spine VLT switches running out of ports