Just talking about "frequencies" may be a misleading way to interpret "channel" disposition in the digital spread-spectrum "world", Gary.

When operating, any given receiver of an XD-V "channel" continuously interrogates its "mated" transmitter so that both of them can seamlessly "hop" between four separate frequencies in order to maintain uninterrupted reception of the robust stream of digital data that contains an encrypted "map" of our precious audio.

Broadly speaking, as I understand it, V75's have access to a full fourteen of these simultaneous "channels", whereas V70's can only operate within the first twelve of the fourteen and V30's only within the first six.

(A given transmitter will only be able to perform such "modeling" as pertains to its own "V series", regardless of whether it's being received by a V30, V70 or V75.)

As to an actual number of simultaneous "systems" that are possible, here's where things become a little weird.

I run two kits of four V70's.

Due to an operational oversight, I've inadvertently had BOTH kits spend a successful day working in close proximity to each other ("battery save" low power, ten metres apart, only separated by a vinyl-laminated timber divider that created two smaller conference venues from one large one), ...with each quartet assigned to (apparently "the same") channels one to four.

Huh ???

Here's what happens.

Each transmitter has its own "identity", a bit like a cell phone handset's "IMEI". When switched on, each one performs a "handshake" with the receiver on its nominal "channel" and from then on (until switch off or "drop out") that's the source recognised by that particular transmitter.

A second transmitter coming up on the same "channel" may limit the range of the first if brought close to that first receiver, but won't be allowed a recognition "handshake" until the first switches off, "drops out" or is actually swamped by more power from (and/or closer proximity to) the second.

Personally, I wouldn't deliberately set out to rely upon more than one system per channel.

Which makes the answer to "...can I operate 26 systems by combining them?" a definite, unequivocal "maybe" !