It does not depend on the number, but on the energy the batteries contain together. You calculate that energy in watt-hours (Wh): voltage (V) times capacity (Ah). A power tool battery of 18 V and 5 Ah thus holds 90 Wh, an e-bike battery of 36 V and 14 Ah about 500 Wh.
A cabinet certified to VDMA 24994 has a maximum battery energy in kWh stated by the manufacturer. Add up the watt-hours of all batteries and stay below that limit: a 10 kWh cabinet takes, for example, around 110 power tool batteries of 90 Wh, but only about twenty e-bike batteries. The maximum load of the cabinet also applies; for the Asecos ION-LINE it is 600 kg.
Belgian rules set no number themselves; the rest follows from your risk assessment. Asecos recommends using a safety cabinet or a fire-separated room from six batteries upwards. Email us which batteries you want to store, and we will work out the right cabinet. See our safety cabinets for batteries.