According to the Young King Ours vol.4, the eighth volume of Hellsing Ultimate DVD & Blu-ray will be released on June 22nd. It had been announced to be released this Spring at Comiket 79.

Trailer of Hellsing Ultimate vol.8 at Comiket 79 (Youtube)

According to the official website, the OVA series will be concluded by the tenth volume. Volumes 8 - 10 will be produced by two studios; Graphinica (the former digital section of GONZO) and studio Kelmadick (the subcontractor of "Shugo Chara!" series and "Genshiken 2").

The changes of the production studios of Hellsing:
TV Series: GONZO
OVA vol.1 - 4: Satelight
OVA vol.5 - 7: Madhouse
OVA vol.8 - 10: Graphinica & Kelmadick

Sources: Scan of YK Ours, Hellsing OVA official website

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Update on May 25th
According to the official website, the release of the eighth volume will be postponed until July 27th. An extra short anime titled "Hellsing: The Dawn" will be included in the volume.

Source: Geneon, Amazon Japan

Hellsing: The Dawn on MAL