Logo retention measures how many customers you kept, ignoring revenue impact. (Customers at end - new customers) / (Customers at start). It's the count-side complement to GRR (which is dollar-based). A company can have high dollar-based GRR but low logo retention if it loses many small customers and keeps the big ones.
(Customers at end - new customers acquired) / Customers at startStarted with 100 customers, ended with 95 (after losing 8 and gaining 3). Logo retention = (95 - 3) / 100 = 92%.
See how public B2B SaaS companies disclose this metric, with full historical time series.