How many prospects do you actually need?

Fewer than you think. A single warmed inbox sending 25–40 cold emails a day, in four-email sequences, works through roughly 150–200 new prospects a month — so a database of a few hundred well-chosen companies covers a quarter of sending. List size is a function of sending capacity and sales target, not ambition, and building for the next 60–90 days beats building for the year.

What actually determines list size?

Sending capacity, at the bottom of everything. The volume discipline that protects deliverability caps each inbox at 25–40 cold emails a day, and each prospect receives a sequence of 4 emails over 14 days — so most of any day's sends are follow-ups, not first touches. Divide through and one inbox starts roughly 6–10 new prospects a day: call it 150–200 a month. That number, not the size of your market, is the rate at which a list gets consumed.

So the question "how big should the database be?" decomposes into two smaller ones: how many conversations does the sales target require, and how many inboxes are running? The assembly process that turns the answer into verified records is in The B2B Database Building Guide; this article is about getting the number right before you build.

How do you work backwards from a revenue target?

The mechanism, with the assumptions kept visible:

1. Start with clients needed per month. Say two.
2. When you know clients, estimate positive replies needed. Suppose — and this is an assumption to replace with your own data — one in five positive replies eventually becomes a client. Two clients then need about ten positive replies a month.
3. When you know replies, size the monthly prospect requirement. On a tight list, around 4% of prospects reply positively (below 3% means fix the campaign, not enlarge the list). Ten replies at 4% is roughly 250 prospects contacted per month.
4. When you know the monthly rate, size the build. 250 a month exceeds one inbox's 150–200, so this target needs two inboxes — and a 60–90 day build of roughly 500–750 verified prospects.

Every number after step one is an estimate to be replaced by your own measured rates within a quarter. But even as estimates they make one thing obvious: nobody doing this arithmetic concludes they need 10,000 contacts.

Why not build a huge list upfront?

Three reasons, in descending order of cost. Decay: B2B contact data goes stale at a rate commonly put at 2–3% a month, so month-ten records from a January build are unreliable by the time you reach them — plan for decay rather than pretending storage preserves value. Learning: the first hundred replies reshape your ICP; a giant upfront build locks in your least-informed targeting. Cash: a £950 standalone database build sized to a quarter, repeated as needed, spends the same money after the learning instead of before it.

A big list also tempts a worse behaviour: raising volume to "get through it", which is how volume discipline dies and deliverability follows.

How does this interact with sub-vertical campaigns?

The arithmetic runs per campaign, not per firm. If you run one campaign per sub-vertical, each needs its own batch — typically a few hundred companies — and its own slice of sending capacity. Remember too that at 5–50 staff, one company yields one prospect: you email the owner of the number, not three hedged job titles, so company count equals prospect count. Some sub-verticals also split structurally: a recruitment agency, for instance, runs a two-sided pipeline — clients on one side, candidates on the other — and each side is its own list and its own arithmetic.

When do you build the next batch?

On a trigger, not a feeling: when the unsent remainder of a list falls below about 30 days of sending, the next build starts. That lead time covers sourcing, enrichment and verification without pausing campaigns, and it keeps every record you send to recently verified. A standing rhythm — build a quarter's worth, send it, rebuild — beats both the panic build and the warehouse.

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Next step: the Growth System Audit — £450, seven days, credited against any build — maps where your growth system leaks and what to build first.