Your example uses the form G is generated by X number of Ys, with complicated forms of X and Y.
Let's look at another (equivalent) example first for simplicity: juice is generated by five number of machines. Although there are instances where this form is used, e.g. "My land runs 5,000 head of cattle." (- Wikipedia), the form is more often written simply as juice is generated by five machines, dropping the phrase "number of".
Returning to your example, you would simply have G is generated by X Ys, or with your original terms, G is generated by (2k+t) 7-cycles. I have inserted brackets to reduce ambiguity, but the presence of two mathematical expressions so close to each other is non-ideal.
Number is awkward in this context since it doesn't identify any part of the unit (7-cycle). If these 7-cycles are periodic, try G is generated by 2k+t periods of 7-cycles. Otherwise, choose some identifying aspect of the 7-cycles to use instead of periods.
Finally, a note about plurality. "head of cattle" (with the singular form head) is a specialist term in this context. In the form X 'count(s)' of Ys, the word used in place of count(s) should normally be singular if X=1 and plural otherwise. The word used in place of Ys should normally be plural. If we used the simpler form X Ys, then we would use a singular form for Ys if X=1, and a plural form otherwise.
Returning to the juice example, the following are all grammatically correct:
- Juice is generated by one machine.
- Juice is generated by five machines.
- Juice is generated by one group of machines.
- Juice is generated by five groups of machines.
In your case, if you retain the algebraic expression 2k+t, then periods should be always be plural.