Is one policy ever enough?

Rarely for on-site or data-touching work. Most engagements need at least two of the above.

What limits should I require?

Follow your client's requirement or a defensible baseline for the risk level.

Policy	Covers	When to require
Public liability	Injury / damage to third parties	Any customer-facing or on-site work
Employers' liability	Injury / illness to employees	Any UK employer (usually mandatory)
Professional indemnity	Financial loss from advice / services	Consultants, agencies, professional services
Product liability	Harm caused by a product	Manufacturers, distributors
Cyber insurance	Breach costs, ransomware, business interruption	Data-touching vendors
Motor / plant	Vehicles and heavy plant	Field services, construction
Directors & officers	Personal liability of directors	Corporate governance