A lot of this depends on the type of leads you are generating. If it is from Social Media, you can use native software (i.e. Facebook Lead tracking) that will do the work for you. However, finding a CRM that can utilize lead tracking from a number of sources regularly can be superior to spreadsheets. There are lower cost companies such as Pipedrive that will track all sources of leads and create profiles for every lead that comes in to the system. Software and apps such as this can actually allow you to track the number of leads, connect it to your own spreadsheets for data tracking (Via something like Zapier) and one up your service by providing a space for your clients to directly reach out to their leads via the software.