The population of Wessex at the time has been hypothesised at being around 2.5 million by Hill and Rumble et al in The Defence of Wessex: The Burghal Hidage and Anglo-Saxon Fortifications. With Alfred's far reaching military reforms and the foundation of county militias called fyrds, it's thought that up to a tenth of the population could theoretically have been "under arms" at any given time although this should not be taken to mean there was an English army of 250,000 going around, as many of these would have been stood down to carry out farming, and many others would be occupied maintaining roads, bridges and fortifications.

A tenth-century document called The Burghal Hidage lists all of the burhs, or fortified government sites, that were in Wessex at the time of Alfred's son, Edward, and tells us the number of 'hides' of land necessary to maintain the garrison of each, with the assumption that a 'hide' provides enough food to equip one man. Winchester, for example, was one of the biggest with 2,400 hides, as was Warwick in Mercia. Exeter had 734, and Wareham in Dorset had 1,600. Overall more than 33 burhs are listed. Other burhs were built throughout the Midlands and Kent.

These burhs were designed to be mutually supportive, and their garrisons would combine to defeat regional threats. In 910, Æthelred of Mercia led a force made up of several Mercian garrisons to defeat a Viking invasion at Tettenhall. From the figures of the Burghal Hidage, this is likely to have been a substantial force, but probably not more than 5 or 6 thousand.

For comparrison, we know Viking longboats could typically have a crew of about 52, and indeed a recent mass grave found near Weymouth with 54 skeletons (but only 51 heads!) is thought to have been a marauding Viking crew that was successfully stopped by the local militia. The Anglo-Saxon Chronicle references great Viking fleets of over 100 ships, which suggest that large Viking maritime forces landing in England also most likely numbered around 5-6 thousand men.